Basic Concepts in Modal Logic1
Edward N. Zalta
Center for the Study of Language and Information
Stanford University
Table of Contents
Preface
Chapter 1 – Introduction
§1: A Brief History of Modal Logic
§2: Kripke’s Formulation of Modal Logic
Chapter 2 – The Language
Chapter 3 – Semantics and Model Theory
§1: Models, Truth, and Validity
§2: Tautologies Are Valid
§2: Tautologies Are Valid (Alternative)
§3: Validities and Invalidities
§4: Validity With Respect to a Class of Models
§5: Validity and Invalidity With Repect to a Class
§6: Preserving Validity and Truth
Chapter 4 – Logic and Proof Theory
§1: Rules of Inference
§2: Modal Logics and Theoremhood
§3: Deducibility
§4: Consistent and Maximal-Consistent Sets of Formulas
§5: Normal Logics
§6: Normal Logics and Maximal-Consistent Sets
Chapter 5 –Soundness and Completeness
§1: Soundness
§2: Completeness
Chapter 6 – Quantified Modal Logic
§1: Language, Semantics, and Logic
§2: Kripke’s Semantical Considerations on Modal Logic
§3: Modal Logic and a Distinguished Actual World
1 Copyright
c 1995, by Edward N. Zalta. All rights reserved.
1
Preface
These notes were composed while teaching a class at Stanford and studying the work of Brian Chellas (Modal Logic: An Introduction, Cambridge:
Cambridge University Press, 1980), Robert Goldblatt (Logics of Time and
Computation, Stanford: CSLI, 1987), George Hughes and Max Cresswell
(An Introduction to Modal Logic, London: Methuen, 1968; A Companion to Modal Logic, London: Methuen, 1984), and E. J. Lemmon (An
Introduction to Modal Logic, Oxford: Blackwell, 1977). The Chellas text
influenced me the most, though the order of presentation is inspired more
by Goldblatt.2
My goal was to write a text for dedicated undergraduates with no
previous experience in modal logic. The text had to meet the following
desiderata: (1) the level of difficulty should depend on how much the
student tries to prove on his or her own—it should be an easy text for those
who look up all the proofs in the appendix, yet more difficult for those
who try to prove everything themselves; (2) philosophers (i.e., colleagues)
with a basic training in logic should be able to work through the text
on their own; (3) graduate students should find it useful in preparing for
a graduate course in modal logic; (4) the text should prepare people for
reading advanced texts in modal logic, such as Goldblatt, Chellas, Hughes
and Cresswell, and van Benthem, and in particular, it should help the
student to see what motivated the choices in these texts; (5) it should link
the two conceptions of logic, namely, the conception of a logic as an axiom
system (in which the set of theorems is constructed from the bottom up
through proof sequences) and the conception of a logic as a set containing
initial ‘axioms’ and closed under ‘rules of inference’ (in which the set of
theorems is constructed from the top down, by carving out the logic from
the set of all formulas as the smallest set closed under the rules); finally,
(6) the pace for the presentation of the completeness theorems should
be moderate—the text should be intermediate between Goldblatt and
Chellas in this regard (in Goldblatt, the completeness proofs come too
quickly for the undergraduate, whereas in Chellas, too many unrelated
2 Three
other texts worthy of mention are: K. Segerberg, An Essay in Classical
Modal Logic, Philosophy Society and Department of Philosophy, University of Uppsala,
Vol. 13, 1971; and R. Bull and K. Segerberg, ‘Basic Modal Logic’, in Handbook of
Philosophical Logic: II , D. Gabbay and F. Günthner (eds.), Dordrecht: Reidel, 1984l;
and Johan van Benthem, A Manual of Intensional Logic, 2nd edition, Stanford, CA:
Center for the Study of Language and Information Publications, 1988.
2
facts are proved before completeness is presented).
My plan is to fill in Chapter 5 on quantified modal logic. At present
this chapter has only been sketched. It begins with the simplest quantified modal logic, which combines classical quantification theory and the
classical modal axioms (and adds the Barcan formula). This logic is
then compared with the system in Kripke’s ‘Semantical Considerations
on Modal Logic’. There are interesting observations to make concerning
the two systems: (1) a comparison of the formulas valid in the simplest
QML that are invalid in Kripke’s system, (2) a consideration of the metaphysical presuppositions that led Kripke to set up his system the way he
did, and finally, (3) a description of the techniques Kripke uses for excluding the ‘offending’ formulas. Until Chapter 5 is completed, the work
in the coauthored paper ‘In Defense of the Simplest Quantified Modal
Logic’ (with Bernard Linsky) explains the approach I shall take in filling
in the details. The citation for this paper can be found toward the end of
Chapter 5.
Given that usefulness was a primary goal, I followed the standard
procedure of dropping the distinguished worlds from models and defining
truth in a model as truth at every world in the model. However, I think
this is a philosophically objectionable procedure and definition, and in
the final version of the text, this may change. In the meantime, the
work in my paper ‘Logical and Analytic Truths that are not Necessary’
explains my philosophical objections to developing modal logic without
a distinguished actual world. The citation for this paper also appears at
the end of Chapter 5.
The class I taught while writing this text (Philosophy 169/Spring
1990) was supposed to be accessible to philosophy majors with only an
intermediate background in logic. I tried to make the class accessible
to undergraduates at Stanford who have had only Philosophy 159 (Basic
Concepts in Mathematical Logic). Philosophy 160a (Model Theory) was
not presupposed. As it turned out, most of the students had had Philosophy 160a. But even so, they didn’t find the results repetitive, since
they all take place in the new setting of modal languages. Of course, the
presentation of the material was probably somewhat slow-paced for the
graduate students who were sitting in, but the majority found the pace
about right. There are fifteen sections in Chapters 2, 3, and 4, and these
can be covered in as little as 10 and as many as 15 weeks. I usually covered
about a section (§) of the text in a lecture of about an hour and fifteen
3
minutes (we met twice a week). Of course, some sections go more quickly,
others more slowly. As I see it, the job of the instructor using these notes
is to illustrate the definitions and theorems with lots of diagrams and to
prove the most interesting and/or difficult theorems.
I would like to acknowledge my indebtedness to Bernard Linsky, who
not only helped me to see what motivated the choices made in these logic
texts and to understand numerous subtleties therein but who also carefully read the successive drafts. I am also indebted to Kees van Deemter,
Christopher Menzel, Nathan Tawil, Greg O’Hair, and Peter Apostoli.
I’m also indebted Guillermo Badı́a Hernández for pointing out some typographical errors (including errors of omission). Finally, I am indebted
to the Center for the Study of Language and Information, which has provided me with office space and and various other kinds of support over
the past years.
4
Chapter One: Introduction
Modal logic is the study of modal propositions and the logical relationships that they bear to one another. The most well-known modal propositions are propositions about what is necessarily the case and what is
possibly the case. For example, the following are all modal propositions:
It is possible that it will rain tomorrow.
It is possible for humans to travel to Mars.
It is not possible that: every person is mortal, Socrates is a person,
and Socrates is not mortal.
It is necessary that either it is raining here now or it is not raining
here now.
A proposition p is not possible if and only if the negation of p is
necessary.
The operators it is possible that and it is necessary that are called ‘modal’
operators, because they specify a way or mode in which the rest of the
proposition can be said to be true. There are other modal operators,
however. For example, it once was the case that, it will once be the case
that, and it ought to be the case that.
Our investigation is grounded in judgments to the effect that certain
modal propositions logically imply others. For example, the proposition
it is necessary that p logically implies the proposition that it is possible
that p, but not vice versa. These judgments simply reflect our intuitive
understanding of the modal propositions involved, for to understand a
proposition is, in part, to grasp what it logically implies. In the recent
tradition in logic, the judgment that one proposition logically implies
another has been analyzed in terms of one of the following two logical
relationships: (a) the model-theoretic logical consequence relation, and
(b) the proof-theoretic derivability relation. In this text, we shall define
and study these relations, and their connections, in a precise way.
§1: A Brief History of Modal Logic
Modal logic was first discussed in a systematic way by Aristotle in De
Interpretatione. Aristotle noticed not simply that necessity implies possibility (and not vice versa), but that the notions of necessity and possibility
5
were interdefinable. The proposition p is possible may be defined as: not-p
is not necessary. Similarly, the proposition p is necessary may be defined
as: not-p is not possible. Aristotle also pointed out that from the separate
facts that p is possible and that q is possible, it does not follow that the
conjunctive proposition p and q is possible. Similarly, it does not follow
from the fact that a disjunction is necessary that that the disjuncts are
necessary, i.e., it does not follow from necessarily, p or q that necessarily
p or necessarily q. For example, it is necessary that either it is raining or
it is not raining. But it doesn’t follow from this either that it is necessary
that it is raining, or that it is necessary that it is not raining. This simple
point of modal logic has been verified by recent techniques in modal logic,
in which the proposition necessarily, p has been analyzed as: p is true in
all possible worlds. Using this analysis, it is easy to see that from the fact
that the proposition p or not-p is true in all possible worlds, it does not
follow either that p is true in all worlds or that not-p is true in all worlds.
And more generally, it does not follow from the fact that the proposition
p or q is true in all possible worlds either that p is true in all worlds or
that q is true in all worlds.
Aristotle also seems to have noted that the following modal propositions are both true:
If it is necessary that if-p-then-q, then if p is possible, so is q
If it is necessary that if-p-then-q, then if p is necessary, so is q
Philosophers after Aristotle added other interesting observations to this
catalog of implications. Contributions were made by the Megarians, the
Stoics, Ockham, and Pseudo-Scotus, among others. Interested readers
may consult ‘the Lemmon notes’ for a more detailed discussion of these
contributions.3
Work in modal logic after the Scholastics stagnated, with the exception
of Leibniz’s suggestion there are other possible worlds besides the actual
world. Interest in modal logic resumed in the twentieth century though,
when C. I. Lewis began the search for an axiom system to characterize
‘strict implication’.4 He constructed several different systems which, he
3 See Lemmon, E., An Introduction to Modal Logic, in collaboration with D. Scott,
Oxford: Blackwell, 1977.
4 See C. I. Lewis, ‘Implication and the Algebra of Logic’, Mind (1912) 12: 522–31; A
Survey of Symbolic Logic, Berkeley: University of California Press, 1918; and C. Lewis
and C. Langford, Symbolic Logic, New York: The Century Company, 1932.
6
thought, directly characterized the logical consequence relation. Today,
it is best to think of his work as an axiomatization of the binary modal
operation of implication. Consider the following relation:
p implies q =df Necessarily, if p then q
Lewis defined five systems in the attempt to axiomatize the implication
relation: S1 – S5 . Two of these systems, S4 and S5 are still in use today.
They are often discussed as candidates for the right logic of necessity
and possibility, and we will study them in more detail in what follows.
In addition to Lewis, both Ernst Mally and G. Henrik von Wright were
instrumental in developing deontic systems of modal logic, involving the
modal propositions it ought to be the case that p.5 This work, however,
was not model-theoretic in character.
The model-theoretic study of the logical consequence relation in modal
logic began with R. Carnap.6 Instead of considering modal propositions,
Carnap considered modal sentences and evaluated such sentences in state
descriptions. State descriptions are sets of simple (atomic) sentences, and
an simple sentence ‘p’ is true with respect to a state-description S iff ‘p’
∈ S. Carnap was then able to define truth for all the complex sentences
of his modal language; for example, he defined: (a) ‘not-p’ is true in S
iff ‘p’ 6∈ S, (b) ‘if p, then q’ is true in S iff either ‘p’ 6∈ S or ‘q’ ∈ S, and
so on for conjunctive and disjunctive sentences. Then, with respect to a
collection M of state-descriptions, Carnap essentially defined:
The sentence ‘Necessarily p’ is true in S if and only if for every
state-description S 0 in M, the sentence ‘p’ is true in S 0
So, for example, if given a set of state descriptions M, a sentence such as
‘Necessarily, Bill is happy’ is true in a state description S if and only if
the sentence ‘Bill is happy’ is a member of every other state description in
M. Unfortunately, Carnap’s definition yields the result that iterations of
the modal prefix ‘necessarily’ have no effect. (Exercise: Using Carnap’s
definition, show that the sentence ‘necessarily necessarily p’ is true in a
state-description S if and only if the sentence ‘necessarily p’ is true in S.)
5 See
E. Mally, Grundgesetze des Sollens: Elemente der Logik des Willens, Graz:
Lenscher and Lugensky, 1926; and G. H. von Wright, An Essay in Modal Logic,
Amsterdam: North Holland, 1951. These systems are described in D. Føllesdal and
R. Hilpinen, ‘Deontic Logic: An Introduction’, in Hilpinen [1971], 1–35 [1971].
6 See R. Carnap, Introduction to Semantics, Cambridge, MA: Harvard, 1942; Meaning and Necessity, Chicago: University of Chicago Press, 1947.
7
The problem with Carnap’s definition is that it fails to define the truth
of a modal sentence at a state-description S in terms of a condition on S.
As it stands, the state description S in the definiendum never appears in
the definiens, and so Carnap’s definition places a ‘vacuous’ condition on
S in his definition.
In the second half of this century, Arthur Prior intuitively saw that
the following were the correct truth conditions for the sentence ‘it was
once the case that p’:
‘it was once the case that p’ is true at a time t if and only if p is
true at some time t0 earlier than t.
Notice that the time t at which the tensed sentence ‘it was once the case
that p’ is said to be true appears in the truth conditions. So the truth
conditions for the modal sentence at time t are not vacuous with respect
to t. Notice also that in the truth conditions, a relation of temporal
precedence (‘earlier than’) is used.7 The introduction of this relation
gave Prior flexibility to define various other tense operators.
§2: Kripke’s Formulation of Modal Logic
The innovations in modal logic that we shall study in this text were developed by S. Kripke, though they were anticipated in the work of S. Kanger
and J. Hintikka.8 For the most part, modal logicians have followed the
framework developed in Kripke’s work. Kripke introduced a domain of
possible worlds and regarded the modal prefix ‘it is necesary that’ as a
quantifier over worlds. However, Kripke did not define truth for modal
sentences as follows:
‘Necessarily p’ is true at world w if and only if ‘p’ is true at every
possible world.
7 See
A. N. Prior, Time and Modality, Westport, CT: Greenwood Press, 1957.
S. Kripke, ‘A Completeness Theorem in Modal Logic’, Journal of Symbolic
Logic 24 (1959): 1–14; ‘Semantical Considerations on Modal Logic’, Acta Philosophica Fennica 16 (1963): 83-94; S. Kanger, Provability in Logic, Dissertation, University of Stockholm, 1957; ‘A Note on Quantification and Modalities’, Theoria 23
(1957): 131–4; and J. Hintikka, Quantifiers in Deontic Logic, Societas Scientiarum
Fennica, Commentationes humanarum litterarum, 23 (1957):4, Helsingfors; ‘Modality
and Quantification’, Theoria 27 (1961): 119–28; Knowledge and Belief: An Introduction to the Logic of the Two Notions, Ithaca: Cornell University Press, 1962.
8 See
8
Such a definition would have repeated Carnap’s error, for it would have
defined the truth of a modal sentence at a world w in terms of a condition
that is vacuous on w. Such a definition collapses the truth conditions of
‘necessarily p’ and ‘necessarily necessarily p’, among other things. Instead,
Kripke introduced an accessibility relation on the possible worlds and this
accessibility relation played a role in the definition of truth for modal
sentences. Kripke’s definition was:
‘Necessarily p’ is true at a world w if and only if ‘p’ is true at every
world w0 accessible from w.
The idea here is that not every world is modally accessible from a given
world w. A world w can access a world w0 (or, conversely, w0 is accessible
from w) just in case every proposition that is true at w0 is possibly true at
w. If there are propositions that are true at w0 but which aren’t possibly
true at w, then that must be because w0 represents a state of affairs that
is not possible from the point of view of w. So a sentence ‘necessarily p’
is true at world w so long as ‘p’ is true at all the worlds that are possible
from the point of view of w.
This idea of using an accessibility relation on possible worlds opened
up the study of modal logic. In what follows, we learn that this accessibility relation must have certain properties (such as reflexivity, symmetry,
transitivity) if certain modal sentences are to be (logically) true. In the
remainder of this section, we describe the traditional conception of modal
logic as it is now embodied in the basic texts written in the past thirtyfive years. These works usually begin with an inductive definition of a
language containing certain ‘proposition letters’ (p, q, r, . . .) as atomic sentences. Complex sentences are then defined and these take the form ¬ϕ
(‘it is not the case that ϕ’), ϕ → ψ (‘if ϕ, then ψ’), and ϕ (‘necessarily ϕ’), where ϕ and ψ are any sentence (not necessarily atomic). Other
sentences may be defined in terms of these basic sentences.
The next step is to define models or interpretations for the language.
A model M for the language is typically defined to be a triple hW, R, Vi,
where W is a nonempty set of possible worlds, R the accessibility relation, and V a valuation function that assigns to each atomic sentence p a
set of worlds V(p). These models allow one to define the model-theoretic
notions of truth, logical truth, and logical consequence. Whereas truth
and logical truth are model-theoretic, or semantic, properties of the sentences of the language, logical consequence is a model-theoretic relation
9
among sentences. A sentence is said to be logically true, or valid, just in
case it is true in all models, and it is said to be valid with respect to a
class C of models just in case it is valid in every model in the class.
The proof theory proceeds along similar lines. Rules of inference relate
certain sentences to others, indicating which sentences can be inferred
from others. A logic Σ is defined to be a set of sentences (which may
contain some ‘axioms’ and) which is closed under the rules of inference
that define that logic. A theorem of a logic is simply a sentence that is
a member of Σ. A logic Σ is said to be sound with respect to a class of
models C just in case every sentence ϕ that is a theorem of Σ is valid
with respect to the class C. And a logic Σ is said to be complete with
respect to a class C of models just in case every sentence ϕ that is valid
with respect C is a theorem of Σ. Such is the traditional conception of
modal logic and we shall follow these definitions here.
10
Chapter Two: The Language
(1) Our first task is to define a class of very general modal languages each
of which is relativized to a set of atomic formulas. To do this, we let the
set Ω be any non-empty set of atomic formulas, with a typical member of
Ω being pi (where i is some natural number). Ω may be finite (in which
case, for some n, Ω = {p1 , p2 , . . . , pn }) or infinite (in which case, Ω =
{pi |i ≥ 1} = {p1 , p2 , p3 , . . .}). The main requirement is that the members
of Ω can be enumerated. We shall use the variables p, q and r to range
over the elements of Ω.
(2) For any given set Ω, we define by induction the set of formulas based
on Ω as the smallest set Fml (Ω) satisfying the following conditions:
.1)
.2)
.3)
.4)
.5)
p ∈ Fml (Ω), for every p ∈ Ω
⊥ ∈ Fml (Ω)
If ϕ ∈ Fml (Ω), then (¬ϕ) ∈ Fml (Ω)
If ϕ, ψ ∈ Fml (Ω), then (ϕ → ψ) ∈ Fml (Ω)
If ϕ ∈ Fml (Ω), then (ϕ) ∈ Fml (Ω)
(3) Finally, we define the modal language based on Ω (in symbols: ΛΩ ) =
Fml (Ω). It is sometimes useful to be able to discuss the subformulas of a
given formula ϕ. We therefore define ψ is a subformula of ϕ as follows:
.1) ϕ is a subformula of ϕ.
.2) If ϕ = ¬ψ, ψ → χ, or ψ, then ψ (χ) is a subformula of ϕ.
.3) If ψ is a subformula of χ and χ is a subformula of ϕ, then ψ is
a subformula of ϕ.
Remark : We read the formula ⊥ as ‘the falsum’, ¬ϕ as ‘it is not the case
that ϕ’, ϕ → ψ as ‘if ϕ, then ψ’, and ϕ as ‘necessarily, ϕ’. In general,
we use the variables ϕ, ψ, χ, θ to range over the formulas in ΛΩ . We drop
the parentheses in formulas when there is little potential for ambiguity,
and we employ the convention that → dominates both ¬ and . So, for
example, the formula ¬p → q is to be understood as (¬p) → q, and the
formula p → q is to be understood as (p) → q. Finally, we define
the truth functional connectives & (‘and’), ∨ (‘or’), and ↔ (‘if and only
if’) in the usual way, and we define ♦ϕ (‘possibly ϕ’) in the usual way
as ¬¬ϕ. Again we drop parentheses with the convention that the order
of dominance is: ↔ dominates →, → dominates & and ∨, and these last
11
two dominate ¬, , and ♦. So, for example, the formula p & ♦p → q is
to be understood as (p & ♦p) → q.
Note that we could do either without the formula ⊥ or without formulas of the form ¬ϕ. The formula ⊥ will be interpreted as a contradiction.
We could have taken any formula ϕ and defined ⊥ as ϕ & ¬ϕ. Alternatively, we could have defined ¬ϕ as ϕ → ⊥. These equivalences are
frequently used in developments of propositional logic. It is sometimes
convenient to have both ⊥ and ¬ϕ as primitives of the language when
proving metatheoretical facts, and that is why we include them both as
primitive. And when it is convenient to do so, we shall sometimes assume
that formulas of the form ϕ & ψ and ♦ϕ are primitive as well.
(4) We define a schema to be a set of sentences all having the same form.
For example, we take the schema ϕ → ϕ to be: {ϕ → ϕ|ϕ ∈ ΛΩ }. So
the instances of this schema are just the members of this set. Likewise
for other schemata. Typically, we shall label schemata using an upper
case Roman letter. For example, the schema ϕ → ϕ is labeled ‘T’.
However, it has been the custom to label certain schemata with numbers.
For example, the schema ϕ → ϕ is labeled ‘4’. In what follows,
we reserve the upper case Roman letter ‘S’ as a variable to range over
schemata.
12
Chapter Three: Semantics and Model Theory
§1: Models, Truth, and Validity
(5) A standard model M for a set of atomic formulas Ω shall be any triple
hW, R, Vi satisfying the following conditions:
.1) W is a non-empty set,
.2) R is a binary relation on W, i.e., R ⊆ (W × W),
.3) V is a function that assigns to each p ∈ Ω a subset V(p) of W; i.e.,
V : Ω → P(W) (where P(W) is the power set of W).
Remark 1 : For precise identification, it is best to refer to the first member
of a particular model M as WM , to the second member of M as RM , and
the third member of M as VM . For any given model M, we call WM
the set of worlds in M, RM the accessibility relation for M, and VM the
valuation function for M. Since all of the models we shall be studying
are standard models, we generally omit reference to the fact that they
are standard. Note that the notion of a model M is defined relative to
a set of atomic formulas Ω. The model itself assigns a set of worlds only
to atomic formulas in Ω. In general, the context usually makes it clear
which set of atomic formulas we are dealing with, and so we typically
omit mention of the set to which M is relative.
Remark 2 : How should we think of the accessibility relation R? One
intuitive way (using notions we have not yet defined) is to suppose that
Rww0 (i.e., w has access to w0 , or w0 is accessible from w) iff every
proposition p true at w0 is possibly true at w. The idea is that what
goes on at w0 is a genuine possibility from the standpoint of w and so
the propositions true at w0 are possible at w. This idea suggests another
way of thinking about accessibility. We can think of the set of all worlds
in W as the set of all worlds that are possible in the eyes of God. But
from the point of view of the inhabitants of a given world w ∈ W, not
all worlds w0 may be possible. That is, there may be truths of w0 which
are not possible from the point of view of w. The accessibility relation,
therefore, makes it explicit as to which worlds are genuine possible worlds
from the point of view of a given world w, namely all the worlds w0 such
that Rww0 . Intuitively, then, whenever Rww0 , if ϕ is true at w0 then
13
♦ϕ is true at w, and just as importantly, if ϕ is true at w, ϕ is true at
w0 . Our definition of truth at below will capture these intuitions.
Example: Let Ω = {p, q}. Then here is an example of a model M for Ω.
Let WM = {w1 , w2 , w3 }. Let RM = {hw1 , w2 i, hw1 , w3 i}. Let VM (p) =
{w1 , w2 }, and VM (q) = {w2 }. We can now draw a picture (note that p
has been placed in the circle defining w whenever w ∈ VM (p)).
Remark 3 : The indexing of WM , RM , and VM when discussing a particular model M is sometimes cumbersome. Since it is usually clear in
the context of discussing a particular model M that W, R, and V are
part of M, we shall often suppress their index.
(6) We now define ϕ is true at world w in model M (in symbols: |=M
w ϕ)
as follows (suppressing indices):
.1) |=M
w p iff w ∈ V(p)
M
.2) 6|=M
w ⊥ (i.e., not |=w ⊥)
M
.3) |=M
w ¬ψ iff 6|=w ψ
M
M
.4) |=M
w ψ → χ iff either 6|=w ψ or |=w χ
0
0
M
.5) |=M
w ψ iff for every w ∈ W, if Rww , then |=w0 ψ
We say ϕ is false at w in M iff 6|=M
w ϕ.
Example: Let us show that the truth conditions of (p → q) at a world
w are not the same as the truth conditions of p → q at w. We can
do this by describing a model and a world where the former is true but
the latter is not. Note that we need a model for the set Ω = {p, q}. The
previous example was actually chosen for our present purpose, so consider
M and w1 as specified in the previous example. First, let us see whether
M
0
0
|=M
w1 (p → q). By (6.5), |=w1 (p → q) iff for every w ∈ W, if Rw1 w ,
M
then |=w0 p → q. Since Rw1 w2 and Rw1 w3 , we have to check both w2
and w3 to see whether p → q is true there. Well, since w2 ∈ V(q), it
M
follows by (6.1) that |=M
w2 q, and then by (6.4) that |=w2 p → q. Moreover,
M
M
since w3 6∈ V(p), 6|=w3 p (6.1), and so |=w3 p → q (6.4). So p → q is true
at all the worlds R-related to w1 . Hence, |=M
w1 (p → q).
Now let us see whether |=M
p
→
q.
By
(6.4), |=M
w1
w1 p → q iff either
M
M
M
6|=w1 p or |=w1 q. That is, iff either 6|=w1 p or for every w0 ∈ W, if
M
Rw1 w0 , then |=M
w0 q (6.5). But, in the example, w1 ∈ V(p), and so |=w1 p
14
0
(6.1). Hence it is not the case that 6|=M
w1 p. So, for every w ∈ W, if
0
M
Rw1 w , then |=w0 q. Let us check the example to see whether this is true.
Since both Rw1 w2 and Rw1 w3 , we have to check both w2 and w3 to see
M
whether q is true there. Well, |=M
w2 q, since w2 ∈ V(q) (6.1). But 6|=w3 q,
M
since w3 6∈ V(q) (6.1). Consequently, 6|=w1 p → q.
So we have seen a model M and world w such that (p → q) is true
at w in M, but p → q is not true at w in M. This shows that the truth
conditions of these two formulas are distinct.
Exercise 1 : Though we have shown that (p → q) can be true while
p → q false at a world, we don’t yet know that the truth conditions of
these two formulas are completely independent of one another unless we
exhibit a model M and world w where p → q is true and (p → q) is
false. Develop such model.
Remark : Note that in the previous example and exercise, we need only
develop a model for the set Ω = {p, q} to describe a world where p → q
is false. We can ignore models for other sets of atomic formulas, and so
ignore what V assigns to any other atomic formula. This should explain
why we didn’t require that the language be based on the infinite set
Ω = {p1 , p2 , . . .}. Had we required that Ω be infinite, then in specifying
(falsifying) models for a given formula ϕ, we would always have to include
a catch-all condition indicating what V assigns to the infinite number of
atomic formulas that don’t appear in ϕ.
Exercise 2 : Suppose that ♦ϕ is a primitive formula of the language. Then
the recursive clause for ♦ϕ in the definition of |=M
w is :
0
0
M
|=M
w ♦ψ iff there is a world w ∈ W such that Rww and |=w0 ψ.
M
Prove that: |=M
w ♦ϕ iff |=w ¬¬ϕ.
Exercise 3 ; Formulate the clause in the definition of |=M
w which is needed
for languages in which ‘&’ is primitive.
Exercise 4 : Suppose that w and w0 agree on all the atomic subformulas
in ϕ and that, in a given model M, for every world u, Rwu iff Rw0 u
M
(i.e., {u | Rwu} = {v | Rw0 v}). Prove that |=M
w ϕ iff |=w0 ϕ.
(7) We define ϕ is true in model M (in symbols: |=M ϕ) as follows:
|=M ϕ =df for every w ∈ W, |=M
w ϕ
15
We say that a schema S is true in M iff every instance of S is true in M.
Example: Look again at the particular model M specified in the above
M
example. We know already that |=M
w1 (p → q). To see whether |=
(p → q) (i.e., to see whether (p → q) is true in M), we need to check
to see whether (p → q) is true at both w2 and w3 . But this is indeed
the case, for consider whether |=M
w2 (p → q). That is, by (6.5), consider
0
whether for every w0 ∈ W, |=M
0
w p → q. In fact, there is no w such that
0
Rw2 w . So, by the failure of the antecedent, indeed, every w0 ∈ WM ,
|=M
w0 p → q. We find the same situation with respect to w3 . So here is an
example of a model M and formula ϕ (= (p → q) ) such that |=M ϕ.
Exercise 1 : Develop a model M such that |=M (p → q) but not because
of any vacuous satisfaction of the definition of truth.
M
Remark : Note that whereas |=M
w ϕ if and only if 6|=w ¬ϕ (by 6.3), it is not
M
M
the case that |= ϕ if and only if 6|= ¬ϕ, though the biconditional does
hold in the left-right direction. To see this, suppose that |=M ϕ. Then
every world w ∈ W is such that |=M
w ϕ, and so by (6.3), every world
¬ϕ.
But
we
know that WM is nonempty (by
w ∈ W is such that 6|=M
w
5.1). So there is a world w ∈ W such that 6|=M
w ¬ϕ, i.e., not every world
M
M
w ∈ W is such that |=w ¬ϕ, i.e., 6|= ¬ϕ. So, by our conditional proof,
if |=M ϕ, then 6|=M ¬ϕ.
However, to see that the converse does not hold, we produce a model
which constitutes a counterexample. Let WM = {w1 , w2 }. Let RM be
{hw1 , w2 i} (though it could be empty). And let VM (p) = {w1 }. Here is
the picture:
M
Note that w1 ∈ V(p), and so |=M
w1 p. Thus, 6|=w1 ¬p. So there is a world
M
w ∈ W such that 6|=w ¬p. But this just means that not every world
M
w ∈ W is such that |=M
¬p. However, since w2 6∈ V(p),
w ¬p, i.e., 6|=
M
6|=w2 p, and so there is a world w ∈ W such that 6|=M
w p. Consequently,
M
M
not every world w ∈ W is such that |=w p. So 6|= p. Thus, we have
a model in which 6|=M ¬p and 6|=M p, which shows that it is not the case
that if 6|=M ¬p, then |=M p.
A similar remark should be made in the case of ϕ → ψ. Note that
M
M
|=M
w ϕ → ψ if and only if the conditional, if |=w ϕ then |=w ψ, holds.
M
However, it is not the case that |= ϕ → ψ if and only if the conditional,
if |=M ϕ then |=M ψ, holds. Again, |=M ϕ → ψ does imply that if |=M ϕ
then |=M ψ, but the conditional if |=M ϕ then |=M ψ does not imply that
|=M ϕ → ψ.
16
Exercise 2 : (a) Prove that |=M ϕ → ψ implies that if |=M ϕ then |=M ψ.
(b) Develop a model that shows that the converse does not hold.
(8) Finally, we now define ϕ is valid (in symbols: |= ϕ) as follows:
|= ϕ iff for every standard model M, |=M ϕ
By convention, we say that a schema S is valid iff every instance of S is
valid.
Example 1 : We show that |= p → (q → p).
Exercise 1 : (a) Suppose that ♦ϕ is a primitive formula of the language,
with the truth conditions specified in (6), Exercise 2 . Show that |= ϕ ↔
¬♦¬ϕ. (b) Show that |= ¬⊥, and |= ¬(ϕ & ¬ϕ).
Example 2 : We show that |= p → (q → p).
Exercise 2 : Show that |= p ↔ ¬¬p.
Exercise 3 : Show that |= ⊥ ↔ (ϕ & ¬ϕ).
Remark 1 : Note the difference between the formulas in Example 1 and
Exercise 1 , on the one hand, and in Example 2 and Exercises 2 and 3 ,
on the other. Whereas the formulas in the first example and exercise are
ordinary modal formulas that are valid, the formulas in Example 2 and
Exercise 2 and 3 are not only valid, they are instances of tautologies in
propositional logic. The validity of these latter formulas does not depend
on the truth conditions for modal operators. Rather, it depends solely on
the truth conditions for the formula ⊥ and the propositional connectives
¬ and →. Note that in Example 2 and Exercises 2 and 3 , we never appeal
to the modal clauses in the proof of validity. The reader should check a
few other modal formulas that have the form of a propositional tautology
to see whether they are valid.
Remark 2 : Here is what we plan to do in the next five sections. In
§2, we work our way towards a proof that the tautologies as a class are
valid, since the evidence suggests that they are. In §3, we study in detail
the realm of valid and invalid schemata. In §4, we next look at some
invalid schemata that nevertheless prove to be valid with respect to certain
interesting classes of models in the sense that the schemata are true in
all the models of the class. In §5, we shall examine why it is that from
the point of view of a certain class C of models, certain schemata are
invalid. Finally, in §6, we investigate some truth and validity preserving
relationships among the formulas of our language.
17
§2: Tautologies are Valid
We turn, then, to the first of our tasks, which is to prove that every tautology is valid. It is important to do this not only for the obvious reason
that propositional tautologies had better be valid, but for a less obvious
one as well. We may think of the set of tautologies (taken as axioms) and
the rule of inference Modus Ponens as constituting propositional logic.
This propositional logic will constitute the basis of all modal logics (and
indeed, we will suppose that it just is the weakest modal logic, for a modal
logic will be defined to be any (possibly null) extension of the axioms and
rules of propositional logic). So if we can show that all the tautologies
are valid, and (in §6) that the rule Modus Ponens preserves validity, we
can show that the propositional basis of modal logic is sound, i.e., that
every theorem derivable from the set of tautologies using Modus Ponens
is valid.
The problem we face first is that we want to distinguish the tautologies
in some way from the rest of the formulas that are valid. We can’t just
use the notion ‘true at every world in every model’, for that is just the
notion of validity. So to prove that the tautologies, as a class, are valid, we
have to distinguish them in some way from the other valid formulas. The
basic idea we want to capture is that the tautologies have the same form
as tautologies in propositional logic. For example, we want to treat the
formula p in p → (¬q → p) as a kind of atomic formula. If we treat
p as the atomic formula r, then p → (¬q → p) would begin to look
like the tautology r → (¬q → r) in propositional logic. So let us think of
p as a quasi-atomic subformula of p → (¬q → p), and then consider
all the ways of assigning truth-values to all the quasi-atomic subformulas
of the language. We can extend each such ‘basic’ assignment to a ‘total’
assignment of truth values to all the formulas in the language, and if ϕ
comes out true in all the total assignments, then ϕ is a tautology. So we
need to define the notions of quasi-atomic formula, and basic assignment,
and then, finally, total assignment, before we can define the notion of a
tautology.
(9) If given a language ΛΩ , we define the set of quasi-atomic formulas in
ΛΩ (in symbols: Ω∗ ) as follows:
Ω∗ = {ϕ | ϕ = p (for some p ∈ Ω) or ϕ = ψ (for some ψ ∈ ΛΩ )}
Let p∗ be a variable ranging over the members of Ω∗ .
18
Example: If we begin with the set Ω = {p, q} of atomic formulas, then the
following are elements of Ω∗ : p, q, p, q, ⊥, p, q, ⊥, . . . , ¬p,
¬q, ¬p, ¬⊥, . . . , (p → p), (⊥ → ⊥), (p → p), . . . , (p →
q), (p → q), . . . , (¬p → p), . . . . The important thing to see here
is that, in addition to genuine atomic formulas, any complex formula
beginning with a is quasi-atomic. Note that there will be only a finite
number of quasi-atomic formulas in any given ϕ.
(10) We next define a basic assignment (of truth values) to be any function
f ∗ defined on Ω∗ which is such that, for any p∗ ∈ Ω∗ , f ∗ (p∗ ) ∈ {T, F }.
Remark : Note that from the set Ω∗ ∪ {⊥} of quasi-atomic formulas, we
can generate every formula of the language ΛΩ by using the connectives
¬ and →. That means that all we have to do to extend f ∗ to all the
formulas of the language is to extend it to formulas of the form ⊥, ¬ψ,
and ψ → χ. Thus, the following recursive definition does give us a total
assignment of truth values to all the formulas ϕ ∈ ΛΩ .
(11) We define a (total) assignment to be any function f defined on ΛΩ
which meets the following conditions:
.1) for some f ∗ , f (p∗ ) = f ∗ (p∗ ), for every p∗ ∈ Ω∗ (i.e., f agrees with
some basic assignment f ∗ of all the quasi-atomic subformulas in Ω∗ )
.2) f (⊥) = F
.3) f (¬ψ) =
(
T , if f (ψ) = F
F , otherwise
(
T , iff either f (ψ) = F or f(χ) = T
.4) f (ψ → χ) =
F otherwise
Whenever f (p∗ ) = f ∗ (p∗ ), we say that that f extends or is based on f ∗ ,
and that f ∗ extends to f . It now follows that if f and f 0 are both based
on f ∗ , then for every ϕ, f (ϕ) = f 0 (ϕ):
(12) Theorem: If f and f 0 are based on the same f ∗ , then, for any ϕ,
f (ϕ) = f 0 (ϕ).
(13) We may now say that a formula ϕ is a tautology iff every assignment
f is such that f (ϕ) = T .
Example: Let us show that ϕ = p → (¬q → p) is a tautology (this
particular ϕ is an instance of the tautology ψ → (¬χ → ψ) ). To show
19
that every f is such that f (ϕ) = T , pick an arbitrary f . Since, p is a
quasi-atomic formula, either f (p) = T or f (p) = F (since f agrees with
some f ∗ ). If the latter, then by (11.4), f (ϕ) = T . If the former, then by
(11.4), f (¬q → p) = T . So again by (11.4), f (ϕ) = T .
Exercise 1 : (a) Show that ¬⊥ is a tautology. (b) Show that (ϕ → ⊥) ↔
¬ϕ is a tautology.
Remark : These examples and exercises show that our definition of a tautology allows us to prove that certain formulas are tautologies. The definition is reasonably simple and serves us well in subsequent work. But, for
arbitrary ϕ, there is no mechanical way of finding arguments such as the
one in the above Example that establish that ϕ is a tautology if indeed
it is. Moreover, you can’t mechanically use the definition to show, for a
given tautology, that indeed it is a tautology, since you can’t check every
assignment f . Even if we start with a language based on the set Ω = {p1 },
it would take a very long time to even specify a basic assignment of the
quasi-atomic subformulas (since, as we have seen, Ω∗ will be an infinite
set). So the definition of ‘tautology’ per se doesn’t offer a mechanical
procedure to discover, for a given ϕ, whether or not ϕ is a tautology,
since strictly speaking, you would have to check an infinite number of
assignments (none of which you can even specify completely).
But we know from work in propositional logic that the truth table
method gives us a mechanical procedure by which we can discover whether
or not a given ϕ is a tautology. Have we lost anything in the move to
modal logic? Actually, we haven’t, for there is a way to construct such a
decision procedure that tests for tautologyhood. Such a procedure will be
described in the Digression that follows (disinterested readers, or readers
who don’t wish to interrupt the train of development of the concepts, may
skip directly ahead to (14) ).
Digression: It is best to intuitively demonstrate our procedure by example
first, and then make it precise it afterwards. Suppose you want to test
whether ϕ in the example immediately above is a tautology. Note that the
following are subformulas of ϕ: p, q, ¬q, p, ¬q → p, and ϕ itself. Now
of all of these subformulas, only five are relevant to the truth functional
analysis of ϕ: q, ¬q, p, q → p, and ϕ itself. Let us call these the
truth-functionally relevant (TFR) subformulas of ϕ. Note that p is not
a TFR-subformula of our particular ϕ, because the truth value of the
subformula of ϕ in which it is contained, namely p, does not depend on
20
p; so p is not relevant to the truth functional analysis of ϕ. And out of
the subformulas that are relevant, only two are quasi-atomic: q and p.
Thus, ϕ’s truth value, from a truth functional point of view, depends just
on the value of q and p. So, really, all of the basic assignment functions
relevant to the truth functional evaluation of ϕ fall into the following four
classes:
F1∗ = {f ∗ |f ∗ (q) = T and f ∗ (p) = T }
F2∗ = {f ∗ |f ∗ (q) = T and f ∗ (p) = F }
F3∗ = {f ∗ |f ∗ (q) = F and f ∗ (p) = T }
F4∗ = {f ∗ |f ∗ (q) = F and f ∗ (p) = F }
In general, if there are n quasi-atomic TFR-subformulas in ϕ, there are
2n different classes of relevant basic assignment functions. Each class
Fi∗ defines a row in a truth table, and each of the quasi-atomic TFRsubformulas defines a column. To complete the truth table, we define a
new column for ¬q, a column for ¬q → p, and finally, a column for
ϕ itself. That is, we define a new column for each of the other TFRsubformulas of ϕ. Now the value in the final column (headed by ϕ) on
row Fi∗ represents the class Fi of all assignments f that (1) agree with a
member f ∗ of Fi∗ on the quasi-atomic TFR-subformulas in ϕ and (2) make
a final truth functional assignment to ϕ by extending f ∗ in a way that
satisfies the definition of f in (11). If we have set things up properly, each
of the assignments f ∈ Fi should agree on the truth value of ϕ (since each
is based on a basic assignment in Fi∗ , all of which agree on the relevant
quasi atomics in ϕ). Consequently, if for every i, each member f of Fi is
such that f (ϕ) = T (i.e., if the value T appears in every row of the final
column of the truth table), then ϕ is a tautology. This is our mechanical
procedure for checking whether ϕ is a tautology. The reader should check
that T does appear in every row under the column headed by ϕ in the
above example.
Of course, this intuitive description of a decision procedure depends
on our having a precise way to delineate of the truth-functionally relevant
subformulas of ϕ, and on a proof that whenever f and f 0 agree on the
relevant quasi-atomic formulas in ϕ, then they agree on ϕ. The latter
shall be an exercise. For the former, the following definition of truthfunctionally relevant subformula of ϕ should work:
21
1. ϕ is a TFR-subformula of ϕ.
2. If ¬ψ or ψ → χ is a TFR-subformula of ϕ, then ψ and χ are
TFR-subformulas of ϕ
Note that we don’t want to add the clause, if ψ is a TFR-subformula of
ϕ, so is ψ, since ψ is not a TFR-subformula of ψ.
Here, then, is our decision procedure for determining whether ϕ is a
tautology:
1. Determine the set of TFR-subformulas of ϕ (this will be a finite
set, and the set can be determined by applying the definition of
TFR-subformula a finite number of times).
2. Isolate from this class the formulas that are quasi-atomic (this will
also be a finite set).
3. Begin the construction of a truth table, with each quasi-atomic
TRF-subformula heading a column (if there are n quasi-atomic truth
functionally relevant subformulas, there will be 2n rows in the truth
table).
4. Extend the truth table to all the other TFR-subformulas in ϕ (with
ϕ heading the final column), filling in the truth table in the usual
way.
5. If every row under the column marked ϕ is the value T , then ϕ is a
tautology.
There is a way of checking this whole procedure. And that is, after
isolating the quasi-atomic TRF-subformulas of ϕ, find all the ones that
begin with a . Replace each such formula in ϕ with a new atomic formula
not in ϕ, and the result should be a tautology in propositional logic. For
example, the quasi-atomic TFR-subformulas of ϕ = p → (¬q → p)
are q and p. Replace p in ϕ with a new atomic formula not already in
ϕ, say r. The result is: r → (¬q → r), and the reader may now employ
the usual decision procedures of propositional logic to verify that this is
indeed a tautology of propositional logic.
Exercise 2 : Show that if f and f 0 agree on the quasi-atomic TFR-subformulas
of ϕ, then f (ϕ) = f 0 (ϕ).
22
(14) We now identify a particular kind of basic assignment; they are
defined to have a special property which is inherited by any (total) assignment based on them and which plays an important role in the proof
that every tautology is valid. Each language ΛΩ , model M, and world
∗
w ∈ WM determines a unique basic assignment function fw
as follows:
∗
∗
for every p∗ ∈ Ω∗ , fw
(p∗ ) = T iff |=M
w p
∗
We call fw
the basic assignment determined by M and w. Note that
∗
∗
corresponding to fw
, there is a (total) assignment fw (based on fw
) of
every ϕ ∈ ΛΩ . We call fw the total assignment determined by M and w.
(15) Lemma: For any ΛΩ , M, w ∈ WM , and ϕ, fw (ϕ) = T iff |=M
w ϕ.
Proof : By induction on ΛΩ .
(16) Theorem: |= ϕ, for every tautology ϕ.
Proof : Appeal to (15).
Alternative §2: Tautologies are Valid (following Enderton)
In some developments of propositional logic (Enderton’s, for example), the
notion of tautology is: ϕ is a tautology iff ϕ is true in all the extensions of
basic assignments of its atomic subformulas.9 The difference here is that
instead of being defined for all the atomic formulas in the language, basic
assignments f ∗ are defined relative to arbitrary sets of atomic formulas.
The basic assignments for a given formula ϕ will be functions that assign
truth values to every member of the set of atomic subformulas in ϕ. An
extended assignment f is then defined relative to a basic assignment f ∗ ,
and extends f ∗ to all the formulas that can be constructed out of the set
of atomic formulas over which f ∗ is defined. So, for a given formula ϕ, f
extends a given basic assignment f ∗ by being defined on all the formulas
that can be constructed out of the set of atomic subformulas in ϕ. Such f s
will therefore be defined on all of the subformulas in ϕ, including ϕ itself.
The definition of a tautology, then, is: ϕ is a tautology iff for every basic
assignment f ∗ (of the atomic subformulas in ϕ), the extended assignment
f (based on f ∗ ) assigns ϕ the value T .
9 See Herbert Enderton, A Mathematical Introduction to Logic, New York: Academic Press, 1972.
23
One advantage of doing things this way is that for any given formula
ϕ, there will be only a finite number of basic assignments for the atomic
subformulas in ϕ, since there will always be a finite number of atomic
subformulas in ϕ. Whenever there are n atomic subformulas of ϕ, there
will be 2n basic assignment functions for the set of those subformulas.
Thus, our decision procedure for determining whether an arbitrary ϕ is
a tautology will simply be: check all the basic assignments f ∗ for the
subformulas in ϕ to see whether f assigns ϕ the value T .
In this section, we redevelop the definitions of the previous section for
those readers who prefer Enderton’s definition of tautology. The twist is
that we have to define basic assignments relative to a given set of quasi atomic formulas. So for any given ϕ, the basic assignments f ∗ will be defined on the set of quasi-atomic subformulas in ϕ. Then we extend those
basic assignments to total assignments defined on all the formulas constructible from such sets of quasi-atomics (these will therefore be defined
for the subformulas of ϕ and ϕ itself). To accomplish all of this, we need
to define the notions of subformula, quasi-atomic formula, and basic truth
assignment to a set of quasi-atomic formulas, and then, finally, extended
assignment, before we can define the notion of a tautology. Readers who
are not familiar with Enderton’s method, or who have little interest in
seeing how the method is adapted to our modal setting, should simply
skip ahead to §3.
8.5) We begin with the notion of subformula. Given the definition of
subformula in (3), we define, for each ϕ ∈ ΛΩ , the set of subformulas of
ϕ (in symbols: Sub(ϕ)) inductively as follows:
Sub(ϕ) =df {ψ| ψ is a subformula of ϕ}
Example: Consider the tautology ϕ = p → (¬q → p), which is an
instance of the tautology ψ → (χ → ψ). By (3.1), ϕ is a subformula of
ϕ, so ϕ is a member of Sub(ϕ). By (3.2), p and ¬q → p are members
of Sub(ϕ). By (3.2), p is a subformula of p, and so by (3.3), p is a
member of Sub(ϕ). Finally, by (3.2), q is a subformula of ¬q, and so by
(3.3), q is a member of Sub(ϕ). So Sub(ϕ) = {p, q, ¬q, p, (¬q → p), ϕ}.
Note that for any formula ϕ, Sub(ϕ) is a finite set, since there are only
a finite number of steps in the construction of ϕ from its basic atomic
constituents.
9) If given a language ΛΩ , we define the set of quasi-atomic formulas in
ΛΩ (in symbols: Ω∗ ) as follows:
24
Ω∗ = {ϕ | ϕ = p (for some p ∈ Ω) or ϕ = ψ (for some ψ ∈ ΛΩ )}
Let p∗ be a variable ranging over the members of Ω∗ .
Example: If we begin with the set Ω = {p, q} of atomic formulas, then the
following are elements of Ω∗ : p, q, p, q, ⊥, p, q, ⊥, . . . , ¬p,
¬q, ¬p, ¬⊥, . . . , (p → p), (⊥ → ⊥), (p → p), . . . , (p →
q), (p → q), . . . , (¬p → p), . . . . The important thing to see here is
that every complex formula beginning with a is quasi-atomic.
9.5 We now define the the set of quasi-atomic subformulas in ϕ (in symbols: Ω∗ϕ ) as:
Ω∗ϕ = Ω∗ ∩ Sub(ϕ)
Example 1 : If ϕ = p → (¬q → p), then Ω∗ϕ = {p, q, p}. Note that
since Sub(ϕ) is finite, so is Ω∗ϕ .
Example 2 : If ϕ = ϕ → ⊥, then Ω∗ϕ = {ϕ}. Though ⊥ is a subformula
of ϕ, it is not quasi-atomic.
10) Next we define a basic assignment for a set of quasi-atomic formulas.
If given any set Γ∗ of quasi-atomic formulas (i.e., if given any subset Γ∗
of Ω∗ ), we say that f ∗ is a basic assignment function for Γ∗ iff f ∗ maps
every p∗ ∈ Γ∗ to a member of {T, F }. Note that if Γ∗ has n members,
there are 2n basic assignment functions for Γ∗ .
Exercise: Consider ϕ in the above examples. Describe the basic assignment functions for the set Ω∗ϕ .
Remark : Note that from the set Ω∗ϕ ∪ {⊥}, we can always regenerate ϕ by
using the connectives ¬ and →. For example, let ϕ = p → (¬q → p).
Then the set Ω∗ϕ of quasi-atomic formulas in ϕ is, as we saw above, {p, q,
p}. But from q we can generate ¬q, and from ¬q and p we can generate
(¬q → p), and from this latter formula, we can generate p → (¬q →
p) = ϕ. The point of considering this is that we now want to extend
basic assignments of the set Ω∗ϕ to assignment functions that cover all
of the subformulas of ϕ, including ϕ itself. So to define such extended
assignment functions, we need to extend basic assignments of Ω∗ϕ to the
set of formulas generated from Ω∗ϕ ∪ {⊥} using ¬ and →, for ϕ itself will
then be in the domain of such a function. So let us introduce notation to
denote this set.
25
10.5) If given a set of quasi-atomic formulas Γ∗ , we can generate a set of
formulas from Γ∗ ∪{⊥} by using the connectives ¬ and →. Let Fml ∗ (Γ∗ ∪
{⊥}) denote the set of formulas generated from Γ∗ ∪ {⊥} using ¬ and →.
Remark : Note that when Γ∗ = Ω∗ (that is, when the set of quasi-atomic
formulas Γ∗ is the entire set of quasi-atomic formulas for our language
ΛΩ ), then Fml ∗ (Γ∗ ∪ {⊥}) = Fml (Ω) (i.e., = ΛΩ ). In other words, from
Ω∗ ∪ {⊥}, we can generate every formula in our language by using the
connectives ¬ and →.
11) If given a basic assignment f ∗ for a set Γ∗ , we define the extended assignment function f of f ∗ to be the function defined on the set of formulas
generated from Γ∗ ∪ {⊥} using ¬ and → (i.e., defined on Fml ∗ (Γ∗ ∪ {⊥}) )
that meets the following conditions:
.1) f (p∗ ) = f ∗ (p∗ ), for every p∗ ∈ Γ∗ (i.e., f agrees with f ∗ on the
quasi-atomic subformulas in Γ∗ )
.2) f (⊥) = F
(
.3) f (¬ψ) =
T , if f (ψ) = F
F , otherwise
(
T , if either f (ψ) = F or f (χ) = T
.4) f (ψ → χ) =
F otherwise
Note that when f ∗ is a assignment for the set Ω∗ϕ of quasi-atomic formulas
in ϕ, then ϕ is in the domain of f , since given the Remark in (10), ϕ is
in Fml ∗ (Ω∗ϕ ∪ {⊥}).
Remark : Clearly, there is an extended assignment f for every f ∗ . In
addition, however, for f to be well-defined, we need to show that there is
a unique extended assignment f for a given f ∗ :
12 Theorem: Let f ∗ be a assignment of Γ∗ . Then if f and f 0 both extend
f ∗ to all the formulas in Fml ∗ (Γ∗ ∪ {⊥}), then f = f 0 .
Remark : In what follows, we correlate the variables f ∗ and f , and we
sometimes say that f extends f ∗ .
13) Finally, we may say: ϕ is a tautology iff for every basic assignment
f ∗ of Ω∗ϕ , f (ϕ) = T (i.e., iff for every basic assignment of the set of quasiatomic subformulas of ϕ, the extended assignment f assigns ϕ the value
T ).
26
Example: Let us show that ϕ = p → (¬q → p) is a tautology. To show
that every f ∗ is such that f (ϕ) = T , pick an arbitrary f ∗ . Since, p is
a quasi-atomic formula, either f ∗ (p) = T or f ∗ (p) = F . If the latter,
then by (11.4), f (ϕ) = T . If the former, then by (11.4), f (¬q → p) = T .
So again by (11.4), f (ϕ) = T .
Remark : Not only does our definition allow us to prove that a given
formula ϕ is a tautology, it gives us a decision procedure for determining,
for an arbitrary ϕ, whether or not ϕ is a tautology. The set of quasiatomic subformulas of ϕ (Ω∗ϕ ) is finite. Suppose it has n members. Then
we have only to check 2n basic assignments f ∗ and determine, in each
case, whether f assigns ϕ the value T . So our modal logic has not lost
any of the special status that propositional logic has with regard to the
tautologies. Indeed, there is a simple way to show that tautologies in our
modal language correspond with tautologies in propositional language.
An example shows the relationship. Again let ϕ = p → (¬q → p).
Recall the set of quasi-atomics in ϕ is {p, q, p}. Now replace each quasiatomic subformula of ϕ beginning with a by a new propositional letter
that is not a subformula of ϕ, say r. The result is: r → (¬q → r), and
the reader may now use the decision procedures of propositional logic to
verify that this is a tautology in any propositional language that generates
formulas from the set {q, r} by using the connectives ¬ and →.
14) We now identify, relative to each model M and world w, a particular
basic assignment; it is defined to have a special property which is inherited
by any extended based on it and which plays an important role in the proof
that every tautology is valid. For any given ϕ ∈ ΛΩ , each model M and
∗
of the set
world w ∈ W defines a unique basic assignment function fw
∗
Ωϕ of quasi-atomic subformulas in ϕ as follows:
∗
∗
for every p∗ ∈ Ω∗ϕ , fw
(p∗ ) = T iff |=M
w p
∗
We call fw
the basic assignment of Ω∗ϕ determined by M and w. Note
∗
that given fw
, we have defined a unique extended assignment fw which
assigns ϕ a truth value.
∗
15) Lemma: For any ΛΩ , M, w ∈ WM , ϕ, and fw
of Ω∗ϕ , fw (ϕ) = T iff
|=M
w ϕ.
16) Theorem: |= ϕ, for every tautology ϕ.
Proof : Appeal to (15).
27
§3: Validities and Invalidities
We now look at a variety of non-tautologous, but nevertheless valid,
schemata. Valid schemata bear an interesting relationship to the schema
K (= (ϕ → ψ) → (ϕ → ψ)), which we prove to be valid in (17). The
axiom schema K plays a role in defining the weakest normal modal logic
K. A normal modal logic is a modal logic which has the tautologies and
instances of K as axioms, and which has as theorems all of the formulas
derivable from these axioms by using Modus Ponens and the Rule of Necessitation (i.e., the rule: from ϕ, infer ϕ). Note that we distinguish the
axiom K, written in Roman, from the logic K, written in italic; we abide
by this convention of writing names of logics in italics throughout. Now
when we prove in later chapters that the logic K is complete, we show
that every valid formula is a theorem of K. That means that all of the
instances of the other schemata that we prove to be valid in this section
will be theorems of K. In later chapters, we also prove that the logic K
is sound , that is, that every theorem of K is valid. We do this in part
by showing that the axiom K is valid and that the Rule of Necessitation
preserves validity. This, together with our demonstration that the tautologies are valid and that Modus Ponens preserves validity, guarantees
the soundness of K, for there are no other theorems of K besides the tautologies, instances of the axiom K, and the formulas provable from these
axioms by Modus Ponens or the Rule of Necessitation.
(17) Theorem: |= (ϕ → ψ) → (ϕ → ψ).
(18) Theorem: The following schemata are valid:
(ϕ → ψ) → (♦ϕ → ♦ψ)
♦(ϕ → ψ) → (ϕ → ♦ψ)
(ϕ & ψ) ↔ (ϕ & ψ)
ϕ → (ψ → ϕ)
¬♦ϕ → (ϕ → ψ)
¬♦⊥
(♦ϕ ∨ ♦ψ) → ♦(ϕ ∨ ψ)
♦(ϕ ∨ ψ) → (♦ϕ ∨ ♦ψ)
(19) Now that we have looked at a wide sample of valid schemata, let us
look at a sample of invalid ones. To show that a formula ϕ is invalid, we
construct a model M and world w where 6|=M
w ϕ. Such models are called
28
falsifying models. To show that schema S is invalid, we build a falsifying
model for an instance of S. The easiest way to construct a falsifying model
M for ϕ is to build a picture of M. Draw a large rectangle to represent
the set of worlds WM . Then draw a circle to represent the world w where
ϕ is going to be false. If the formula we plan to falsify is the conditional
ϕ → ψ, we suppose that ϕ is true at w (by sticking it in the circle) and
that ψ is false at w (by sticking ¬ψ into the circle). If the formula we
plan to falsify is ϕ, we suppose ¬ϕ is true at w (sticking ¬ϕ into the
circle). After doing this, we fill out the details of the model, by adding
ψ-worlds accessible from w whenever ♦ψ is true at w (and ¬ψ-worlds
when ¬ψ, i.e., ♦¬ψ, is true at w). To add ψ-worlds accessible to w to
our picture, we draw another circle, label it (say as ‘w0 ’), draw an arrow
from w to w0 (to represent the accessibility relationship), and insert ψ
into w0 . Be sure to add χ at every accessible world introduced whenever
χ is true at w. If all we have at w is a formula of the form χ, it is best
to add at least one R-related world where ϕ is true. We proceed in this
fashion until we have reached the atomic subformulas of ϕ. Of course, it
is essential that we do this in enough detail to ensure that we have not
constructed an incoherent description of a model, i.e., a model such that
for some world w, both p and ¬p are true at w. So, if we can build a
coherent picture in which ¬ϕ, we know that there is a model and world
where ϕ is false. So ϕ is not valid. Once we have developed a picture
that convinces us that a formula ϕ is not valid, we can always decode our
picture into a formal description of a model M, and give a formal proof
that there is a world w ∈ W such that 6|=M
w ϕ.
Example 1 : We build a falsifying model for an instance of D (= ϕ → ♦ϕ)
with a single world and the empty accessibility relation.
Remark 1 : Notice that if we were to add any other world w0 and allow
w to access it, the model would become incoherent, for we would have to
add p to w0 (since p is true at w) and ¬p to w0 (since ¬♦p, i.e., ¬p,
is true at w).
Example 2 : We build a falsifying model for an instance of T (with p
and ¬p true at w and p true at accessible world w0 ).
Remark 2 : Notice that the picture would become incoherent were w
accessible from w0 .
(20) Theorem: The following schemata are not valid.
29
(ϕ ∨ ψ) → (ϕ ∨ ψ)
(♦ϕ & ♦ψ) → ♦(ϕ & ψ)
ϕ → ♦ϕ (‘B’)
ϕ → ϕ (‘4’)
♦ϕ → ♦ϕ (‘5’)
♦ϕ → ♦ϕ (‘G’)
(21) Exercise: Determine whether the following are invalid by trying to
construct a falsifying model. Note that if your attempts to produce a
falsifying model always end in incoherent pictures, it may be because ϕ
is valid. Prove that ϕ is invalid, if it is invalid, or valid, if it is valid:
ϕ → ϕ
(¬♦ϕ & ♦ψ) → ♦(¬ϕ & ψ)
♦ϕ → ϕ
(ϕ & ♦ψ) → ♦(ϕ & ψ)
ϕ → ϕ
♦(ϕ → ψ) ∨ (ψ → ϕ)
♦ϕ → ♦ϕ
(♦ϕ → ψ) → (ϕ → ψ)
♦ϕ → ϕ
♦ϕ → ♦ϕ
§4: Validity with respect to a Class of Models and
Validity on Frames and Classes of Frames
In this section, we look at some invalid schemata that nevertheless prove
to be valid with respect to a certain class of models, in the sense of being
true in all the models of a certain class. Some of these formula will be
valid with respect to models in which the set of worlds has a certain size.
However, our principle focus shall be on formulas true in all models in
which the accessibility relation meets a certain interesting condition. The
schema 4 proves to be valid with respect to the class of models having
a transitive accessibility relation. Results of this kind are important for
our work in later chapters. Once we have seen that a schema S is valid
with respect to a certain interesting class C of models, we will be in a
position to show that the normal modal logic based on S (i.e., having S
as an axiom schema) is sound with respect to C, i.e., that every theorem
of the logic based on S is valid with respect to C. For example, we
30
shall prove that every theorem of the normal modal logic K4 is valid
with respect to the class of transitive models (and therefore is sound with
respect to this class). The modal logic K4 has the tautologies, instances
of K, and instances of the 4 schema as axioms, and has as theorems all
of the formulas derivable from these by Modus Ponens and the Rule of
Necessitation. Still later on, when we prove that a normal modal logic
is complete with respect to a class of models C, we show that all the
formulas valid with respect to C are theorems of the logic. For example,
when we show that K4 is complete with respect to the class of transitive
models, we show that the formulas which are valid in the class of transitive
models are theorems of K4 .
(22) Let us now define ϕ is valid with respect to a class C of standard
models (in symbols: C |= ϕ) as follows:
C |= ϕ =df for every M ∈ C, |=M ϕ
We say that a schema S is valid with respect to C iff all of the instances
of S are valid with respect to C.
Remark : Clearly, any formula that is valid simpliciter is valid in every
class of models (i.e., if |= ϕ, then C |= ϕ, for any class C). So the
tautologies and other valid formula we have studied so far are valid with
respect to every class C. However, many of the invalid formulas we’ve
studied prove to be true in all the models of a certain interesting class.
We say ‘interesting’ class because every non-valid non-contradiction is
true in at least some trivial class of models, namely, the class of models in
which it is true. But there are some non-valid non-contradictions that are
valid with respect to the class of all models meeting a certain non-trivial
condition.
Example 1 : We show that ϕ → ϕ is valid with respect to the class of
models having a single world in W. Since we know, for every M, that
WM must have at least one world, we may define the class C1 of single
world models as follows: M ∈ C1 iff for every w, w0 ∈ WM , w = w0 . We
now show that C1 |= ϕ → ϕ. Pick an arbitrary M ∈ C1 and w ∈ WM .
M
M
Either |=M
w ϕ or 6|=w ϕ. If the latter, then |=w ϕ → ϕ. If the former,
0
then suppose that Rww , for some arbitrary w0 . Since M ∈ C1 , we know
that w = w0 . So we know that |=M
w0 ϕ. Consequently, by conditional
0
M
proof, if Rww , then |=w0 ϕ, and since w0 was arbitrary, we know that
M
for every w0 , if Rww0 , then |=M
w0 ϕ. So |=w ϕ, by (6.5). So by (6.4),
31
|=M
w ϕ → ϕ. So, by disjunctive syllogism, it follows in either case that
|=M
w ϕ → ϕ. So since M and w were arbitrarily chosen, C1 |= ϕ → ϕ.
Example 2 : We show that ψ → ϕ and ♦ϕ → ψ are valid with respect
to the class of models in which the accessibility relation is empty, i.e., in
which no worlds are R-related to each other. In such models, for any world
0
0
w, |=M
w ϕ, since it is vacuously true that for every w , if Rww then
M
M
|=w0 ϕ. And so, for any world w, we always find that |=w ψ → ϕ, for
any formula ψ. Moreover, in models with an empty accessibility relation,
it is never the case that there is a w0 such that both Rww0 and |=M
w0 ϕ.
M
So, 6|=M
♦ϕ,
for
any
world
w.
Thus,
for
any
world,
|=
♦ϕ
→
ψ,
for
w
w
any formula ψ. So ♦ϕ → ψ is valid with respect to models in which the
accessibility relation is empty.
Remark 2 : The two examples we just looked at show us how invalid
formulas can be valid with respect to a class of models, where the models
in the class satisfy a certain somewhat interesting condition. In the next
subsection, we look at formulas that are valid with respect to a classes
of models satisfying even more interesting conditions. For example, we
discover that instances of the schema T (= ϕ → ϕ) (which we already
know are invalid) are valid with respect to the class of models in which
the accessibility relation is reflexive. Of course, by redefining the notion
of a model so that all models are stipulated to have reflexive accessibility
relations, it would follow that the T schema is valid simpliciter . But
instead of doing this, we just use the relative definition of validity.
(23) Consider the following list of properties of a binary relation R:10
P1 ) ∀u∃vRuv (‘serial’)
P2 ) ∀uRuu (‘reflexive’)
P3 ) ∀u∀v(Ruv → Rvu) (‘symmetric’)
P4 ) ∀u∀v∀w(Ruv & Rvw → Ruw) (‘transitive’)
P5 ) ∀u∀v∀w(Ruv & Ruw → Rvw) (‘euclidean’)
P6 ) ∀u∀v∀w(Ruv & Ruw → v = w) (‘partly functional’)
P7 ) ∀u∃!vRuv (‘functional’)
10 This
follows Goldblatt [1987], pp. 12-13.
32
P8 ) ∀u∀v(Ruv → ∃w(Ruw & Rwv)) (‘weakly dense’)
P9 ) ∀u∀v∀w(Ruv&Ruw → Rvw∨v = w∨Rwv) (‘weakly connected’)
P10 ) ∀u∀v∀w(Ruv & Ruw → ∃t(Rvt & Rwt)) (‘weakly directed’)
Consider next a corresponding list of schemata:
S1 ) ϕ → ♦ϕ (‘D’)
S2 ) ϕ → ϕ (‘T’)
S3 ) ϕ → ♦ϕ (‘B’)
S4 ) ϕ → ϕ (‘4’)
S5 ) ♦ϕ → ♦ϕ (‘5’)
S6 ) ♦ϕ → ϕ
S7 ) ♦ϕ ↔ ϕ
S8 ) ϕ → ϕ
S9 ) [(ϕ & ϕ) → ψ] ∨ [(ψ & ψ) → ϕ]
(‘L’)
S10 ) ♦ϕ → ♦ϕ (‘G’)
Now consider the following theorem:
Theorem: For any model M, if RM satisfies Pi , then |=M Si (1 ≤ i ≤ 10).
Remark 1 : Note that this theorem in effect says that Si is valid with
respect to the class of Pi -models (i.e., models M in which RM is Pi ).
Consider what the theorem says about P2 and S2 , for example: for every
model M, if RM is reflexive, then |=M ϕ → ϕ. In other words, the T
schema is valid with respect to the class of reflexive models.
Remark 2 : It is an interesting fact that the ‘converse’ of this theorem
is false. Consider what the ‘converse’ says in the case of P2 and S2 : for
any model M, if |=M ϕ → ϕ, then RM is reflexive. To see that this is
false requires some argument. We construct a particular model M1 (for
the language ΛΩ , where Ω = {p}) in which all the instances of the T
schema are true but where RM1 is not reflexive. M1 has the following
components: W = {w1 , w2 }; R = {hw1 , w2 i, hw2 , w1 i}; and V(p) =
{w1 , w2 }. To see that all of the instances of the T schema are true in this
model, we have to first argue that the following is a fact about M1 :
33
M1
1
Fact: |=M
w1 ϕ iff |=w2 ϕ.
Proof : By induction on the construction of ϕ.
Now we use this Fact to argue for the following two claims, for any ϕ: (a)
M1
1
that |=M
ϕ → ϕ.
w1 ϕ → ϕ, and (b) that |=
M1
1
Proof of (a): Clearly, either (i) |=M
w1 ϕ or (ii) 6|=w1 ϕ. Assume
M1
0
(i). Then by our Fact, |=w2 ϕ. So for any w , if Rw2 w0 , then
M1
1
|=M
w0 ϕ. But Rw2 w1 . So |=w1 ϕ. So by conditional proof, if
M1
M1
|=w1 ϕ, then |=w1 ϕ, which means, by the Remark in (7), that
1
|=M
w1 ϕ → ϕ. Now assume (ii). Then, by antecedent failure,
M1
1
|=w1 ϕ → ϕ. So, in either case, we have |=M
w1 ϕ → ϕ.
1
Proof of (b): By (a) and the Fact, we know |=M
w2 ϕ → ϕ. So since
M1
ϕ → ϕ is true in both w1 and w2 , we have |= ϕ → ϕ.
Since it is clear that RM1 is not reflexive, we have established that every
instance of the T schema is true in M1 , but RM1 is not reflexive. This
counterexample shows that the ‘converse’ of the present theorem is false.
Exercise: Show that the ‘converse’ of this theorem is false in the case of
the schema 4 and transitivity; i.e., find a model M in which every instance
of the 4 schema is true but in which RM is not transitive.
Remark 3 : It is interesting that if instead of focusing on classes of models,
we focus on the underlying structure of a model, we can produce an
interesting and true ‘converse’ to our theorem. The underlying structure
of a given model is called a frame.
(24) A frame F is any pair hW, Ri, where W is a non-empty set of
worlds and R is an accessibility relation on W. Again, for precise identification, we refer to the set of worlds in frame F as WF , and refer to
the accessibility relation of F as RF . The only difference between frames
and models is that frames do not have valuation functions V that assign
sets of worlds to the atomic formulas of the language. Frames constitute
the purely structural component of models. We say that the model M
is based on the frame F iff both WM = WF and RM = RF . We may
now define another sense of validity that is relative to a frame: ϕ is valid
on the frame F (in symbols: F |= ϕ) iff for every model M based on F,
|=M ϕ. We say that a schema is valid on frame F iff every instance of the
schema is valid in every model based on F. Clearly, for any given frame
34
F, any formula that is valid simpliciter is valid on F (i.e., if |= ϕ, then
F |= ϕ, for any F). But consider now the following claim, which looks
similar to the ‘converse’ of the theorem in (23), and which does hold for
the Si and Pi in (23):
(25) Theorem: For any frame F, if F |=Si , then RF satisfies Pi .
Remark : In other words, if schema Si is valid on the frame F, then the
accessibility relation of F has the property Pi . Consider what this asserts
with respect to S3 and P3 : for any frame F, if F |= ϕ → ϕ, then RF
is transitive.
Question: Why is it that the claim:
For any model M, if |=M Si , then RM satisfies Pi ,
is false, whereas the claim:
For any frame F, if F |=Si , then RF satisfies Pi ,
is true?
Remark : When we showed in (23) that the T schema is valid wrt the
class of reflexive models, we were showing something about the abstract
structure of the models in the class. The particular valuation function
V to atomic formulas doesn’t really make a difference! Rather it is the
structure, namely the set of worlds plus accessibility relation, that is responsible for the validity of the schema. What this ‘converse’ theorem
tells us is that the validity of the schema relative to just the structure
(frame), guarantees that the accessibility relation of the structure has
the corresponding property. These facts should convince you that the
following holds for the Pi and Si in (23):
(26) Theorem: For any frame F, RF satisfies Pi iff F |=Si (1 ≤ i ≤ 10).
(27) There is one more relativized notion of validity which proves to be
useful. And that is the notion of validity with respect to a class of frames.
Let CF be a class of frames. Then ϕ is valid with respect to the class of
frames CF just in case ϕ is valid on every frame F in CF , i.e., just in
case for every F ∈ CF , F |= ϕ.
Remark : Note that for ϕ to be valid with respect to the class of frames
CF , ϕ must be true in every model M based on any frame in CF . Clearly,
then, any formula that is valid simpliciter is valid with respect to every
class of frames (i.e., if |= ϕ, then CF |= ϕ, for any CF ). Note also that
using this notion of validity, we may read the theorem in (26) in the leftright direction as: Si is valid with respect to the class of all Pi -frames (a
35
Pi -frame is a frame F in which RF satisfies Pi ). Of course, if ϕ is valid
with respect to the class of all Pi -frames, then ϕ is valid with respect
to the class of all Pi -models. The notion of a frame, and of a class of
frames, have been developed in important recent work in modal logic. We
introduce them now so that the reader will start to become familiar with
the rather powerful notion of validity with respect to a class of frames.
§5: Validity and Invalidity with respect to a Class
In this section, we develop our intuitions about the kinds of schemata
that are invalid in certain classes of models (or frames). It is instructive
to see why, for example, the schema 4 is invalid with respect to the class
of reflexive models. These exercises help us to visualize the relationships
between modal schemata and interesting classes of models and frames,
and thus give us a deeper understanding of modality.
(28) Some facts:
.1) The schemata B, 4, and 5 are not valid with respect to the class of
reflexive models (frames)
.2) The schema 4 is not valid with respect to the class of symmetrical
models (frames).
.3) The schema 5 is not valid with respect to the class of transitive
models, nor in the class of symmetric models (frames).
.4) The schemata 4 and 5 are not valid with respect to the class of
reflexive symmetric models (frames).
.5) The schemata B and 5 and not valid with respect to the class of
reflexive transitive models (frames).
.6) The schemata T and B are not valid with respect to the class of
serial transitive euclidean models (frames).
.7) The schema T is not valid with respect to the class of serial symmetric models (frames).
.8) The schema 4 is not valid with respect to the class of serial euclidean
models (frames).
36
.9) The schema D is not valid with respect to the class of symmetric
transitive models (frames).
Remark 1 : There is a another important reason for proving these facts besides that of developing our intuitions. And that is they play an important
role in establishing the independence of modal logics, i.e., in establishing
that there are theorems of logic Σ that are not theorems of logic Σ0 .
We shall not spend time in the present work investigating such questions
about the independence of logics, but simply prepare the reader for such
a study, indicating in general how these facts play a role. For example we
know that the schema T is valid with respect to reflexive models. In the
next chapter we shall consider the modal logic KT based on the axioms K
and T. And in the final chapter, we shall prove that the logic KT is sound
with respect to the class of reflexive models, i.e., that every theorem ϕ
of KT is valid with respect to the class of reflexive models, i.e., that if ϕ
is not valid with respect to the class of reflexive models, then ϕ is not a
theorem of KT . But by (28.1), the schema 4 is not valid with respect to
the class of reflexive models. So, by the soundness of KT , the schema 4
is not a theorem of KT . This means that any modal logic that contains 4
as a theorem will be a distinct logic, and moreover, that KT is not an an
extension of any logic Σ containing 4, since there are theorems of Σ not
in KT . Similarly, the schemas B and 5 will not be theorems of KT , since
neither of these is valid in the class of reflexive models.
Consider, as a second example, (28.2). The fact that the schema 4
is not valid in the class of symmetric models can be used to show that
4 is not a theorem of the modal logic KB (the modal logic based on the
axioms K and B), since once it is shown that KB is sound with respect
to the class of symmetric models, it follows that any schema not valid in
the class of symmetric models is not a theorem of KB .
Remark 2 : From (28.4), we discovered that the schemata 4 and 5 were
both invalid with respect to the class of reflexive symmetric models. Note
that we can produce a single model in which both 4 and 5 are false. In
such models, we need only show that 4 is false at one world and that 5
is false at another world. To do this, it suffices to show that an instance
of 4, say p → p, is false at one world, whereas an instance of 5, say
♦q → ♦q (or even ♦¬p → ♦¬p), is false at another world. Here is a
model that works:
Exercise: Develop a reflexive transitive model that falsifies both B and 5,
37
and develop a serial transitive euclidean model that falsifies both T and
B.
(29) Some facts about relations:
.1) If relation R is reflexive, R is serial.
.2) A symmetric relation R is transitive iff it is euclidean.
.3) A relation R is reflexive, symmetric, and transitive iff R is reflexive
and euclidean iff R is serial, symmetric, and transitive iff R is serial,
symmetric, and euclidean.
.4) If a relation R is symmetrical or euclidean, then R is weakly directed.
.5) If a relation R is euclidean, it is weakly connected.
.6) If a relation R is functional, it is serial.
Remark : The reason for studying facts of this kind is that they help us
to show that a given logic Σ is an extension of another logic Σ0 , once the
soundness of Σ and the completeness of Σ0 are both established. Take
(29.1), for example. The fact that every reflexive relation is serial implies
that the class of reflexive models is a subset of the class of serial models.
So any formula ϕ valid in the class of all serial models is valid in the
class of reflexive models, i.e., (a) if C-serial|= ϕ , then C-refl|= ϕ. In later
chapters, we discover (b) that the modal logic KD is sound with respect
to the class of all serial models in the sense that the theorems of KD are
all valid with respect to the class of serial models, and (c) that the modal
logic KT is complete with respect to the class of reflexive models in the
sense that the formulas valid with respect to the class of reflexive models
are all theorems of KT . In other words, we prove (b) if `KD ϕ, then Cserial|= ϕ (here the symbols ‘`KD ϕ’ mean that ϕ is a theorem of KD),
and (c) if C-refl|= ϕ, then `KT ϕ. So, putting (b), (a), and (c) together,
it follows that if `KD ϕ, then `KT ϕ (i.e., that every theorem of KD is
a theorem of KT ). This means that the logic KT is an extension of the
logic KD. So facts about the accessibility relation R of the present kind
will eventually help us to establish interesting relationships about modal
systems.
Exercise: Find other entailments between the properties of relations P1
– P10 defined in (23).
38
§6: Preserving Validity and Truth
We conclude this chapter with some results that prepare us for the following chapter on logic and proof theory. These results have to do
with validity- and truth-preserving relationships among certain formulas. These relationships ground the rules of inference defined in the next
chapter. It is appropriate to demonstrate now that the relationships that
serve to justify the rules of inference are indeed validity and truth preserving, for this shows that the rules of inference based on these relationships
themselves preserve validity and truth. Recall that to say that a modal
logic Σ is sound (wrt a class C of models) is to say that every theorem
of Σ is valid (with respect to C). Recall also that the theorems of a
logic are the formulas derivable from its axioms using its rules of inference. We’ve already seen how some of the formulas that will be taken
as axioms prove to be valid with respect to certain classes of models. So
by showing that the rules of inference associated with (normal) modal
logics preserve validity and truth, we show that such logics never allow
us to derive invalidities from validities already in it, nor falsehoods from
non-logical truths we might want to add to the logic.
There are four rules of inference that will play a significant role in
Chapters 3 and 4. In this section, we describe these rules only informally,
relying on the readers past experience with such rules to understand what
role they play in logic. The four rules we shall study may be described as
follows:
Modus Ponens: From ϕ → ψ and ϕ, infer ψ.
Rule of Propositional Logic (RPL): From ϕ1 , . . . , ϕn , infer ψ, whenever ψ is a tautological consequence of ϕ1 , . . . , ϕn (the notion of
‘tautological consequence’ will be defined shortly).
Rule of Necessitation (RN): From ϕ, infer ϕ.
Rule K (RK): From ϕ1 → . . . → ϕn → ψ, infer ϕ1 → . . . →
ϕn → ψ.
In RK, the formula ϕ1 → . . . → ϕn → ψ is an abbreviation of the formula
ϕ1 → (. . . → (ϕn → ψ) . . .), and the formula ϕ1 → . . . → ϕn → ψ is
an abbreviation of the formula ϕ1 → (. . . → (ϕn → ψ) . . .).
We now examine whether these rules preserve validity and truth.
39
(30) Consider the relationship among the formulas ϕ, ϕ → ψ, and ψ
established by the following theorem:
Theorem: If |= ϕ → ψ and |= ϕ, then |= ψ.
Remark : This relationship among formulas grounds the rule of inference
Modus Ponens (MP). MP allows us to regard ψ as a theorem of a modal
logic whenever ϕ → ψ and ϕ are theorems of that logic. The present
(meta-)theorem tells us that MP preserves validity. Note that in proving
this theorem, we in effect show that Modus Ponens also preserves truth
in a model and truth at a world in a model. That is, we have shown both:
(a) if |=M ϕ → ψ and |=M ϕ, then |=M ψ, and (b) if |=M
w ϕ → ψ and
M
ψ.
ϕ,
then
|=
|=M
w
w
Exercise: Show that MP preserves validity with respect to a class of
models, validity on a frame, and validity with respect to a class of frames.
That is, show: (a) for any class of standard models C, if C |= ϕ → ψ and
C |= ϕ, then C |= ψ, (b) for any frame F, if F |= ϕ → ψ and F |= ϕ, then
F |= ψ, and (c) for any class of frames CF , if CF |= ϕ → ψ and CF |= ϕ,
then CF |= ψ.
Remark : Note that we can form a ‘corresponding conditional’ by conjoining the hypotheses of the rule into an antecedent and the conclusion of
the rule into the conclusion of a condtional. The corresponding conditional for MP is: ((ϕ → ψ) & ϕ) → ψ. Question: Is the corresponding
conditional for the rule MP valid?
(31) The following definitions will prepare the ground for showing that
a very special rule of inference is valid. We say that ψ is a tautological
consequence of formulas ϕ1 , . . . , ϕn iff for every assignment function f , if
f (ϕ1 ) = T and . . . and f (ϕn ) = T , then f (ψ) = T .11 By convention, if
n = 0, then ψ is a tautologous consequence of the empty set of formulas
ϕ1 , . . . , ϕn just in case every f is such that f (ψ) = T . The following are
straightforward consequences of these definitions:
11 If you have read Alternative §2 and prefer the Enderton method of defining assignments, then you should define tautological consequence a little differently. Let Γ
be a finite of sentences, say {ϕ1 , . . . , ϕn }. Then, we define the set of quasi-atomic
subformulas in Γ (in symbols: Ω∗Γ ) as:
Ω∗Γ = Ω∗ϕ1 ∪ . . . ∪ Ω∗ϕn .
Now we may say: ψ is a tautological consequence of ϕ1 , . . . , ϕn iff for every basic
assignment f∗ of the set Ω∗Γ∪{ψ} , if f(ϕ1 ) = T and . . . and f(ϕn ) = T , then f(ψ) = T .
40
.1) ψ is a tautological consequence of ϕ iff ϕ → ψ is a tautology.
.2) ψ is a tautological consequence of ϕ1 , . . . , ϕn iff ϕ1 → . . . → ϕn → ψ
is a tautology.
.3) If ϕ1 , . . . , ϕn are all tautologies, and ψ is a tautological consequence
of ϕ1 , . . . , ϕn , then ψ is a tautology.
.4) ϕ is a tautology iff ϕ is a tautological consequence of the empty set
of formulas ϕ1 , . . . , ϕn when n = 0.
.5) ϕ → (ψ → χ) and (ϕ & ψ) → χ are tautological consequences of
each other, and in general, ϕ1 → . . . → ϕn → ψ and (ϕ1 & . . . &ϕn ) →
ψ are tautological consequences of each other.
.6) ⊥ and ϕ & ¬ϕ are tautological consequences of each other.
.7) ψ is a tautological consequence of ϕ1 , . . . , ϕn , and ϕ1 → . . . → ϕn →
ψ.
(32) Theorem: Let ψ be a tautological consequence of ϕ1 , . . . , ϕn . Then,
if |= ϕ1 and . . . and |= ϕn , then |= ψ.
Proof : Use (31.2), (16), and apply (30) n-times.
Remark : This relationship among formulas grounds the Rule of Propositional Logic (RPL). This rule will allow us to conclude, when ψ is a
tautological consequence of ϕ1 , . . . , ϕn , that ψ is a theorem of a modal
logic whenever ϕ1 , . . . , ϕn are theorems of the logic. The present (meta-)theorem tells us that RPL preserves validity.
Exercise 1 : Show that truth in a model and truth at a world in a model
are both preserved whenever ψ is a tautological consequence of ϕ1 , . . . , ϕn .
Exercise 2 : Show that validity with respect to a class of models, validity
on a frame, and validity with respect to a class of frames is preserved
whenever ψ is a tautological consequence of ϕ1 , . . . , ϕn . That is, show:
(a) for any class of standard models C, if C |= ϕ1 and . . . and C |= ϕn ,
then C |= ψ, whenever ψ is a tautological consequence of ϕ, (b) for any
frame F, if F |= ϕ1 , and . . . and F |= ϕn , then F |= ψ, whenever ψ
is a tautological consequence of ϕ; and (c) for any class of frames CF ,
if CF |= ϕ1 and . . . and CF |= ϕn , then CF |= ψ, whenever ψ is a
tautological consequence of ϕ.
41
Remark : The corresponding conditional for RPL is: (ϕ1 & . . . &ϕn ) → ψ,
where ψ is a tautological consequence of ϕ1 , . . . , ϕn . Question: Is the
corresponding conditional for RPL valid, i.e., is |= (ϕ1 & . . . & ϕn ) → ψ,
whenever ψ is a tautological consequence of ϕ1 , . . . , ϕn ?
(33) Consider the following relationship between ϕ and ϕ:
Theorem: If |= ϕ, then |= ϕ
Remark : This relationship grounds the Rule of Necessitation (RN). It
proves to be important to the definition of ‘normal’ modal logics. RN
allows us to suppose that ϕ is a theorem of a normal modal logic whenever ϕ is a theorem of the logic. The present (meta-)theorem tells us that
RN preserves validity.
Exercise 1 : Show that this rule preserves truth in a model but not truth
at a world in a model.
Remark : Note that the corresponding conditional for RN, ϕ → ϕ, is
not valid (as we showed in (20)). However, the corresponding conditional
of the two previous rules we’ve examined are valid. For example, the corresponding conditional for MP, ((ϕ → ψ) & ϕ) → ψ, is valid. Moreover,
the corresponding conditional for RPL, (ϕ1 & . . . & ϕn ) → ψ (when ψ is a
tautological consequence of ϕ1 , . . . , ϕn ), is valid . Question: What is the
difference among these rules that accounts for the different properties of
their corresponding conditionals?
Exercise 2 : Show that RN rule preserves validity with respect to a class
of models, validity on a frame, and validity with respect to a class of
frames.
(34) Consider one final relationship among formulas:
Theorem: If |= ϕ1 → . . . → ϕn → ψ, then
|= ϕ1 → . . . → ϕn → ψ
Proof : (Contributed by Chris Menzel): First prove:
(*) |= (ϕ1 → . . . → ϕn → ψ) → (ϕ1 → . . . → ϕn → ψ), for any
ϕ1 , . . . , ϕn , ψ
by induction:
The base case n = 0 is trivial and it is easiest to consider
n = 1 a base case as well, which falls out immediately by the
42
validity of K. So suppose the theorem holds for some arbitrary
n (n ≥ 1). For an arbitrary model M and world w, and any
formulas θ1 , . . . , θn+1 , χ, assume:
(1) |=M
w (θ1 → . . . → θn+1 → χ)
We want to show:
(ϑ) |=M
w (θ1 → . . . → θn+1 → χ)
By the validity of K, it follows from (1) that
(2) |=M
w θ1 → (θ2 → . . . → θn+1 → χ)
By the induction hypothesis, we have:
(3) |=M
w (θ2 → . . . → θn+1 → χ) →
(θ2 → . . . → θn+1 → χ)
By the validity of hypothetical syllogism, it follows from (2)
and (3) that:
(4) |=M
w θ1 → (θ2 → . . . → θn+1 → χ)
But if you drop the parenthesis from (4) and remove the superflous θ2 , we have (ϑ).
Now assume |= ϕ1 → . . . → ϕn → ψ. Then since RN preserves validity,
we have |= (ϕ1 → . . . → ϕn → ψ). Then by (*) and the fact that MP
preserves validity, we have |= ϕ1 → . . . → ϕn → ψ.
Remark : This relationship grounds the Rule RK. It is a rule that characterizes normal modal logics, and it allows us to suppose that ϕ1 →
. . . → ϕn → ψ is a theorem of a normal modal logic whenever
ϕ1 → . . . → ϕn → ψ is a theorem of the logic. The present (meta-)theorem tells us that RK preserves validity.
Exercise 1 : Prove by induction that the previous theorem holds for the
corresponding conditional, i.e., that if |= (ϕ1 & . . . & ϕn ) → ψ, then
|= (ϕ1 & . . . & ϕn ) → ψ. Hint: Use (32).
Exercise 2 : Show that rule RK preserves truth in a model, but not truth
at a world in a model. Question: Is the corresponding conditional for this
rule valid? Why not?
43
Exercise 3 : Show that rule RK preserves validity with respect to a class
of models, validity on a frame, and validity with respect to a class of
frames. That is, show: (a) for any class of models C, if C |= ϕ1 → . . . →
ϕn → ψ, then C |= ϕ1 → . . . → ϕn → ψ. (b) for any frame F,
if F |= ϕ1 → . . . → ϕn → ψ, then F |= ϕ1 → . . . → ϕn → ψ;
and (c) for any class of frames CF , if CF |= ϕ1 → . . . → ϕn → ψ, then
CF |= ϕ1 → . . . → ϕn → ψ.
44
Chapter Four: Logic and Proof Theory
We now work our way towards a definition of a logic and its theorems.
Given the anticipatory remarks we have been making in the previous
chapter, the reader may expect the following traditional definitions:
A logic is a set of axioms plus some rules of inference. A
proof is any sequence of formulas which is such that every
member of the sequence either (a) is an axiom, or (b) follows
from previous members of the sequence by a rule of inference.
A theorem is any formula ϕ for which there is a proof. The
symbol ‘` ϕ’ is employed to denote that ϕ is a theorem.
This conception of logic, which we refer to in what follows as “the conception of a logic as an axiom system,” has no room for unaxiomatizable
logics.12
However, in the present work, we do not adopt this conception of a
logic. The recent work in modal logic employs a slightly different conception, one that allows for the possibility of unaxiomatizable logics. For
some readers, the notion we develop may seem unfamiliar, but since it
does appears most often in the recent works of modal logicians, we introduce it here, with the hope that our discussion will prove to be valuable
to the reader when he or she approaches more advanced works in the
field. The first remarkable feature of present conception concerns rules of
inference, which we look at somewhat differently. Instead of having the
form ‘from ϕ1 , . . . , ϕn , infer ψ’, a rule R is simply taken to be a relation
between the hypotheses of the rule and the conclusion. We can use this
relation to talk about sets of formulas that contain the conclusion of rule
R whenever they contain the hypotheses of R. We call such sets ‘closed
under’ the rule R. So rules of inference are relations that allow us to define
certain sets. This brings us to the second remarkable difference about the
present conception. A logic is conceived to be just a set of formulas, and
the set is usually identified by stipulating: (1) that it contains some initial
elements (the ‘axioms’), and (2) that it is closed under certain rules of
12 On this conception, it is standard to require that there be an effective method
for determining whether a formula is an axiom, and that for each rule, there be an
effective method for deciding when sentences are related by the rule as hypotheses and
conclusion. Give these requirements, then, the theorems of a logic may be effectively
enumerated.
45
inference. The theorems of a logic Γ will be defined simply as the members of Γ. Consequently, the present conception doesn’t require that a
logic be axiomatizable. However, it turns out that we shall be interested
primarily in the logics that are axiomatizable, and so one might wonder,
why use this conception of logic?
The principal advantage of this conception is that it allows us to describe, in a much more perspicuous way, the frequently encountered situation in which there are several distinct axiom systems each having the
same set of theorems. For example, there are numerous ways of axiomatizing propositional logic, each of which has the same set of theorems
(namely, the set of tautologies). On the conception of logic as an axiom
system, we would have to say that each of these axiomatizations constitutes a different logic. But on the present conception, we in effect identify
the logic with the set of its theorems. Thus, we may say that there is only
one set, namely, the set of tautologies, that constitutes propositional logic.
And similarly with other logics for which there are distinct axiomatizations.
§1: Rules of Inference
(35) Let us say that a rule of inference R is any relation defined by pair
sequences of the form h{ϕ1 , . . . , ϕn }, ψi (n ≥ 0), where the members of
{ϕ1 , . . . , ϕn } are said to be the hypotheses of R, and ψ the conclusion of
R. For example, the rule Modus Ponens is the relation defined by pair
sequences of the form:
h{ϕ → ψ, ϕ}, ψi.
The Rule of Necessitation is the relation defined by pair sequences of the
form:
h{ϕ}, ϕi.
Hereafter, we shall more simply designate rules as having the following
form: ϕ1 , . . . , ϕn /ψ. So we shall hereafter designate the four rules of
inference previously introduced as follows:
MP: ϕ → ψ, ϕ/ψ
RPL: ϕ1 , . . . , ϕn /ψ (n ≥ 0), where ψ is a tautological consequence
of ϕ1 , . . . , ϕn .
46
RN: ϕ/ϕ
RK: ϕ1 → . . . → ϕn → ψ/ϕ1 → . . . → ϕn → ψ
A set of formulas Γ is said to be closed under rule R just in case Γ contains
the conclusion of R whenever it contains the hypotheses of R (or just
contains the conclusion of R when n = 0 and there are no hypotheses).
So, for example, Γ is closed under Modus Ponens just in case: ψ ∈ Γ
whenever both ϕ → ψ ∈ Γ and ϕ ∈ Γ. Γ is closed under RPL just in case:
if ϕ1 , . . . , ϕn ∈ Γ, then ψ ∈ Γ, whenever ψ is a tautological consequence
of ϕ1 , . . . , ϕn . So to prove Γ is closed under RPL, we typically assume
(a) that ϕ1 , . . . , ϕn ∈ Γ and (b) that ψ is a tautological consequence of
ϕ1 , . . . , ϕn , and then show that ψ ∈ Γ.
Remark 1 : Traditionally, rules of inference are construed in a somewhat
dynamic way. They permit us to infer formulas from other formulas.
This is the way we looked at rules in §6 of Chapter 2. This traditional
understanding goes naturally with the conception of a logic as an axiom
system, for the notion of a proof is basic to that conception. Thus rules
of inference allow us to build proofs of the theorems from the axioms, and
on this conception, the set of theorems is constructed “from the bottom
up”.
But now we are looking at rules a little differently. We are looking at
them as relations between sets of formulas and other formulas. Modus
Ponens, for example, relates sets of the form {ϕ → ψ, ϕ} to the formula
ψ. This is all that is meant by saying that ψ is the consequence of ϕ and
ϕ → ψ by MP. RPL, for example, relates sets of the form {ϕ1 , . . . , ϕn }
to those formulas ψ which are tautological consequences of ϕ1 , . . . , ϕn . In
the next section, we define a logic, in general, to be any set which contains
certain initial axioms and which is closed under certain rules of inference.
This views the set of theorems (i.e., the logic itself) as constructed “from
the top down”. The logic is carved out from the set of all formulas.
Later in this chapter, particular logics will be defined as the smallest
set containing the instances of certain schemata and closed under certain
rules. For example, the logic K is later identified as the smallest set
which contains the tautologies, the instances of the K schema, and which
is closed under the rules MP and RN. This, too, carves out the logic K
“from the top down”, for we may think of this as arriving at the set K
by paring down all the sets which contain the tautologies, contain the K
axiom, and are closed under MP and RN, until we reach the smallest one.
47
Remark 2 : Note that RPL has a special feature that distinguishes it from
the other rules: the conclusion ψ of RPL need not be related in any formal
way to the hypotheses ϕ1 , . . . , ϕn . In particular, the conclusion does not
have to be a subformula of one of the hypotheses, nor do subformulas
of the conclusion have to appear in the hypotheses. For example, the
sequence ⊥/ϕ & ¬ϕ constitutes an instance of RPL, since the conclusion
is a tautological consequence of the hypothesis. This is unlike the other
rules, such as MP and RN. In MP, one of the subformulas of a hypothesis
appears as the conclusion. In RN, a subformula of the conclusion appears
as a hypothesis. These facts mean that, unlike the other rules, RPL may
be applied even in the case when there are no hypotheses, as long as the
conclusion is a tautological consequence of the empty set of hypotheses.
This in fact may happen, though it can’t happen in the case of MP, RN,
or RK, since such rules cannot be applied when there are no hypotheses.
Remark 3 : On the present conception of logic, we may think of a propositional logic as any set that contains all the tautologies and which is closed
under MP. Using this definition, lots of sets will qualify as propositional
logics. But in what follows, we let propositional logic per se be the smallest set containing all tautologies and closed under MP. This is just the set
of tautologies (we prove this below). Our next task is to define the notion
of a modal logic, and the notion we want is this: a modal logic is any extension (possibly even the null extension) of a propositional logic. Since
‘contains all tautologies and is closed under MP’ is a perfectly adequate
definition of a propositional logic, we simply use this as the definition of
a modal logic. So once we define a modal logic as any set containing all
the tautologies and closed under MP, it should turn out that the set of
tautologies is the weakest modal logic, i.e., it should turn out that every
modal logic is an extension of the set of tautologies. We turn now to the
definitions and theorems that yield these consequences.
§2: Modal Logics and Theoremhood
(36) We say that a set of sentences is a modal logic just in case it contains
every tautology and is closed under the rule MP.
(37) Theorem: PL (= {ϕ | ϕ is a tautology }) is a modal logic.
(38) Theorem: Γ is a modal logic iff Γ is closed under RPL.
48
Remark 2 : This theorem tells us that the sets that both contain every
tautology and are closed under MP are precisely the sets closed under
RPL. This means not only that RPL is a ‘derived’ rule (in the sense
that every modal logic ‘obeys’ or is closed under this rule), but also that
‘closure under RPL’ constitutes an equivalent definition of a modal logic.
We shall exploit these facts on many occasions in what follows, for we
can now establish that a set Γ is a modal logic simply by showing that Γ
is closed under RPL. This often proves to be more efficient than showing
that Γ contains all the tautologies and is closed under MP.13
(39) In what follows we use the variable Σ to range over sets of sentences
that qualify as modal logics. If Σ is a modal logic, then we say ϕ is a
theorem of Σ (in symbols: `Σ ϕ) iff ϕ ∈ Σ. In terms of this definition,
we can make our talk of one logic being the extension of another logic
more precise. Whenever Σ and Σ0 are modal logics, we say that Σ0 is a
Σ-logic (or an extension of the logic Σ) just in case for every ϕ, if ϕ ∈ Σ,
then ϕ ∈ Σ0 . In other words, Σ0 is a Σ-logic just in case it contains every
theorem of Σ.
(40) Theorem: (.1) If `PL ϕ, then `Σ ϕ. (.2) Every modal logic Σ is a
PL-logic. (.3) PL is the smallest modal logic.
Remark : At first it may seem odd to think of PL as a modal logic. But
the notion of modal logic we were after is: any extension, including the
null extension, of propositional logic. These theorems show that we have
captured this idea. It therefore does little harm to our understanding of
a modal logic to suppose that PL is one and it simplifies everything if
we do so. There is one nice feature of this understanding of PL. And
that is the soundness and completeness of the propositional basis of each
modal logic is built right in. To say that the propositional logic PL is
sound is to say that every theorem is a tautology, and to say that it is
complete is to say that every tautology is a theorem. Clearly, PL is both
sound and complete—its theorems (i.e., members) just are all and only
the tautologies. So since PL is the embodiment of propositional logic and
forms the basis of every modal logic, every modal logic has a sound and
complete propositional basis.
13 Chellas, in [1980], just uses ‘closure under RPL’ as the definition of a modal logic.
While this definition works perfectly well for the study of propositional modal logics
(which is what is covered in Chellas’ book), it is not as useful for the case of predicate
modal logics. Since we shall investigate basic modal predicate logic in the present text,
we use the more traditional definition.
49
(41) Theorem: The following are modal logics:
.1) {ϕ| |=M
w ϕ}, for every M and w
.2) {ϕ| |=M ϕ}, for every M
.3) {ϕ| |= ϕ}
.4) {ϕ| C |= ϕ}, for every class of models C
.5) {ϕ| F |= ϕ}, for every frame F
.6) {ϕ| CF |= ϕ}, for every class of frames CF
.7) {ϕ|ϕ ∈ ΛΩ }, for each ΛΩ
Remark 1 : In light of (38), we can prove these facts either by showing
that the sets in question contain every tautology and are closed under
MP, or by showing that the sets in question are closed under RPL.
Remark 2 : It is important to see next that some modal logics can be
axiomatized. To axiomatize a logic Σ, it is not enough to find axioms
and rules of inference from which the members of Σ can be generated.
For every logic Σ is trivially axiomatized by the set Σ and the Rule of
Reiteration: ϕ/ϕ. An axiomatization should have two important features:
(a) there should be an effective method of determining whether a given
formula is an axiom, and (b) there should be, for each rule of inference,
an effective method of determining whether formulas are related by the
rule as hypothesis and conclusion. Feature (a) requires that the set of
axioms be ‘decidable’, and that is why {ϕ| |=M
w ϕ} (41.1) typically fails to
be axiomatized by the trivial axiomatization consisting of itself and the
Rule of Reiteration. There is no effective method of determining whether
a formula ϕ is an element of the set of formulas true at w in M (would
that there be such an effective method!). Feature (b) requires that the
rules can be effectively applied. Typically, this requirement is satisfied
by rules in which the conclusion has a certain form that is related to the
form of the hypotheses.
(42) Suppose that Γ is a decidable set, and that rules R1 , R2 , . . . can be
effectively applied. Then we may say that the logic Σ is axiomatized by
the set Γ (of axioms) and the rules of inference R1 , R2 , . . . just in case:
ϕ ∈ Σ iff there is a proof of ϕ from Γ (using the rules R1 , R2 , . . . ). A
proof of ϕ from Γ (using the rules R1 , R2 , . . .) is any sequence of formulas
50
hϕ1 , . . . , ϕn i, with ϕ = ϕn , such that each member of the sequence ϕi
(1 ≤ i ≤ n) either (a) is a member of Γ, or (b) is the conclusion, by one
of the rules (R1 , R2 , . . .) of previous members of the sequence. Note, for
example, that to say that ϕi is a conclusion of previous members of the
sequence by the rule MP is, in precise terms, to say: ∃j, k < i such that
ϕk = (ϕj → ϕi ). To say, for example, that ϕi is a conclusion of previous
members of the sequence by the rule RPL is, in precise terms, to say:
∃j1 , . . . , jk , with 1 ≤ j1 ≤ jk < i, such that ϕi is the conclusion by RPL of
ϕj1 , . . . , ϕjk . Similarly precise statements can be formulated for the other
rules of inference we have discussed so far. Note also that since we have
identified theoremhood with membership, the definition of axiomatized by
guarantees that the notion of theoremhood associated with the present
conception of logic is equivalent to the notion of theoremhood associated
the conception of a logic as an axiom system.
Remark : Clearly, if given a finite list of axioms, or a finite list of schemata
(the instances of which are taken as axioms), then there is an effective
method for determining whether a formula is an axiom. Moreover, for
each of the rules of inference discussed so far, there is an effective method
for determining whether formulas are related as hypotheses and conclusion. This is clearly true in the case of MP, RN, and RK, but it is true
even in the case of RPL. Our work developing a decision procedure that
tests whether a formula is a tautology can easily be turned into a decision procedure that determines whether a formula ψ is a tautological
consequence of ϕ1 , . . . , ϕn . So RPL can be effectively applied as well.
(43) Theorem: (.1) PL is axiomatized by the set of tautologies and the
rule MP. (.2) PL is axiomatized by the empty set ∅ of axioms and the
rule RPL.
Exercise: Find other axiomatizations of propositional logic in other logic
texts, and examine how it is established that these axiomatizations generate all and only tautologies as theorems.
§3: Deducibility
Traditionally, deducibility is linked with axiomatizability via the notion of
a proof. Under the conception of a logic as an axiom system, it is standard
to introduce the notion of deducibility before the notion of theoremhood,
since the former is a slightly more general notion. On that conception,
51
one typically sees: ϕ is deducible in the logic Σ from a set of sentences Γ
(in symbols: Γ `Σ ϕ ) iff there is a proof of ϕ from Σ ∪ Γ. Then the notion
of a theorem would be introduced as a special case: ϕ is a theorem of logic
Σ iff ϕ is derivable in Σ from the empty set ∅ (i.e., `Σ ϕ iff ∅ `Σ ϕ ). From
these definitions, it is possible to prove that the deducibility relation has
all sorts of interesting properties.
However, on the present conception of logic, we employ a different
notion of deducibility. It proves to be equivalent to the traditional notion
in the case of axiomatizable logics. Consider the following definition:
(44) A formula ϕ is deducible (derivable) from a set of sentences Γ in a
modal logic Σ (in symbols: Γ `Σ ϕ) iff there are formulas ϕ1 , . . . , ϕn ∈ Γ
such that Σ contains the theorem ϕ1 → . . . → ϕn → ϕ. Formally:
Γ `Σ ϕ =df ∃ϕ1 , . . . , ϕn ∈ Γ (n ≥ 0) such that `Σ ϕ1 → . . . →
ϕn → ϕ
Remember here that the formula ϕ1 → . . . → ϕn → ϕ is shorthand for
the formula ϕ1 → (. . . (ϕn → ϕ) . . .). Note also that this latter formula is
tautologically equivalent to the defined notation: (ϕ1 & . . . & ϕn ) → ϕ.
The biconditional having these two formulas as the conditions is therefore
a tautology, and so an element of every logic Σ. Thus MP guarantees
that the latter is in Σ iff the former is. So it is an immediate consequence
of our definitions that Γ `Σ ϕ iff ∃ϕ1 , . . . , ϕn ∈ Γ (n ≥ 0) such that
`Σ (ϕ1 & . . . & ϕn ) → ϕ.
In what follows, we write Γ 6 `Σ ϕ whenever it is not the case that
Γ `Σ ϕ.
Remark 1 : Let us think of Γ as a non-logical theory. Then Γ `Σ ϕ
essentially says that ϕ is a derivable consequence of the theory Γ if given Σ
as the underlying logic. For example, one might hold both that p → ¬♦q
and p for non-logical reasons. Our definition should capture the intuition
that ¬♦q is deducible from these two hypotheses in any modal logic Σ.
To see that our definition does capture this intuition, we let Γ1 be {p →
¬♦q, p} and we show Γ1 `Σ ¬♦q. By definition, we need to show that
there are formulas ϕ1 , . . . , ϕn ∈ Γ1 such that ϕ1 → . . . → ϕn → ¬♦q is a
theorem of Σ. To see that there are such formulas, note that (p → ¬♦q)
→ (p → ¬♦q) is a tautology. So `PL (p → ¬♦q) → (p → ¬♦q).
So by (40), `Σ (p → ¬♦q) → (p → ¬♦q). So there are formulas ϕ1
(= p → ¬♦q) and ϕ2 (= p) in Γ1 such that Σ `Σ ϕ1 → (ϕ2 → ¬♦q).
Thus, by (44), Γ1 `Σ ¬♦q.
52
Remark 2 : Deducibility is defined relative to a logic Σ. It would serve
well to give an example of a set of formulas Γ and a formula ϕ such that ϕ
is not derivable from Γ relative to logic Σ but is derivable relative to logic
Σ0 . Here is such an example. Suppose that your theory consists of the
following two propositions: (p → q) and p. Let Γ2 be {(p → q), p}.
Now in propositional logic, one may not validly infer q from Γ2 . This
is captured by our definition, since (p → q) → (p → q) is not a
tautology and so not a theorem of PL. In addition, none of the following
are tautologies: p → q, and (p → q) → q, and q. So none of these
is a theorem of PL. But now we have considered all the combinations (even
the empty combination) of formulas in Γ2 , and we’ve discovered that there
are no formulas ϕ1 , . . . , ϕn ∈ Γ2 such that `PL ϕ1 → . . . → ϕn → q. So
Γ2 6 `PL q, confirming our intuitions.
However, in the modal logic K (which we define formally in a later
section), the instances of the axiom K are theorems. So `K (p → q) →
(p → q). Clearly, then, there are formulas ϕ1 , ϕ2 ∈ Γ2 such that
`K ϕ1 → (ϕ2 → q). So by the definition of deducibility (relative to the
logic K), we have Γ2 `K q, which is what we should expect. Relative to
a logic containing instances of the schema K, we should be able to derive
q from (p → q) and p.
(45) Lemma: If Γ `Σ ϕ, and ψ is a tautological consequence of ϕ, then
Γ `Σ ψ.
(46) Theorem: The definitions of theorem and deducibility have the following consequences (some of which may be more easily proved by using
the previous lemma):14
.1)
.2)
.3)
.4)
.5)
.6)
.7)
`Σ ϕ iff ∅ `Σ ϕ
`Σ ϕ iff for every Γ, Γ `Σ ϕ
If Γ `PL ϕ, then Γ `Σ ϕ
When Σ0 is a Σ-logic, if Γ `Σ ϕ, then Γ `Σ0 ϕ
If ϕ ∈ Γ, then Γ `Σ ϕ
If Γ `Σ ψ and {ψ} `Σ ϕ, then Γ `Σ ϕ
If Γ `Σ ϕ and Γ ⊆ ∆, then ∆ `Σ ϕ
14 For
the most part, this follows Chellas [1980], p. 47, and to a lesser extent, Lemmon
[1977], p. 17. However, we think it important to add the lemma in (45) prior to the
introduction of these facts, for the lemma follows just as directly from the definition
and is slightly more general.
53
.8) Γ `Σ ϕ iff there is a finite subset ∆ of Γ such that ∆ `Σ ϕ
.9) Γ `Σ ϕ → ψ iff Γ ∪ {ϕ} `Σ ψ
Remark : (46.1) tells us that the theorems of Σ are precisely those formulas derivable in Σ from the empty set of sentences; (46.2) says that
the theorems of Σ are precisely those formulas derivable in Σ from every
set of formulas; (46.3) means the deducibility relation in propositional
logic is preserved in all modal logics; (46.4) says the deducibility relation
in Σ is preserved by every extension of Σ; (46.5) says the members of
a set Γ are all derivable from Γ; (46.6) asserts a kind of transitivity of
the deducibility relation; (46.7) says any formula derivable from a set is
derivable from any of its supersets; (46.8) says derivability is compact in
the sense that derivability from a set Γ always implies derivability from
a finite subset of Γ; and finally (46.9) is a ‘deduction theorem’, namely,
that ϕ → ψ is derivable from a set Γ iff ψ is derivable from the enlarged
set Γ ∪ {ϕ} (this follows from (45)).
(47) Generalized Lemma: If Γ `Σ ϕ1 and . . . and Γ `Σ ϕn , and ψ is a
tautological consequence of ϕ1 , . . . , ϕn , then Γ `Σ ψ.
(48) Facts about Deducibility (most of which are immediate consequences
of (45) and (47)):15
.1) {⊥} `Σ ϕ (for all ϕ)
.2) {ϕ, ¬ϕ} `Σ ⊥
.3) {¬ϕ → ⊥} `Σ ϕ
.4) Γ `Σ ϕ & ψ iff Γ `Σ ϕ and Γ `Σ ψ
.5) If Γ `Σ ϕ or Γ `Σ ψ, then Γ `Σ ϕ ∨ ψ
.6) If Γ `Σ ϕ ∨ ψ and Γ `Σ ¬ϕ, then Γ `Σ ψ
.7) If Γ `Σ ϕ → ψ and Γ `Σ ϕ, then Γ `Σ ψ
.8) If Γ `Σ ¬(ϕ → ψ), then Γ `Σ ϕ and Γ `Σ ¬ψ
.9) If Γ `Σ ψ, then Γ `Σ ϕ → ψ
.10) If Γ `Σ ¬ϕ, then Γ `Σ ϕ → ψ
.11) If Γ `Σ ϕ → ψ and Γ `Σ ψ → χ, then Γ `Σ ϕ → χ
.12) Γ `Σ (ϕ & ψ) → χ iff Γ `Σ ϕ → (ψ → χ)
.13) Γ `Σ (ϕ1 & . . . & ϕn ) → ψ iff Γ `Σ ϕ1 → . . . → ϕn → ψ
15 For the most part, this follows Chellas, [1980], p. 50. However, we’ve introduced
here the generalized lemma as the easiest way to prove these facts.
54
(49) Theorem: Let Γ be a set of sentences and Σ a modal logic. Then,
Γ `Σ ϕ iff there is a finite sequence ϕ1 , . . . , ϕm (with ϕm = ϕ) every
member of which either (a) is a member of Σ ∪ Γ, or (b) follows from
previous members by MP.
Remark : This theorem establishes that our definition of deducibility,
which bypasses the notion of a proof altogether, is equivalent to the more
traditional definition in terms of the notion of a proof. It also explains
why the properties of our deducibility relation, demonstrated in (46) and
(48), correspond exactly to the properties had by the deducibility relation
defined in the conception of a logic as an axiom system.
Exercise 1 : Let us say that a set of sentences Γ is deductively closed Σ just
in case every formula that is deducibleΣ from Γ is already in Γ. Formally:
Γ is deductively closed Σ =df for every ϕ, if Γ `Σ ϕ, then ϕ ∈ Γ
Prove that for any set Γ, Γ is deductively closedΣ iff Γ is a Σ-logic.
M
Exercise 2 : (a) Prove that if |=M
w Σ ∪ Γ (i.e., |=w ψ, for ψ ∈ Σ ∪ Γ) and
M
Γ `Σ ϕ, then |=w ϕ. (b) Prove that {ϕ | Γ `Σ ϕ} is the smallest modal
logic containing Σ ∪ Γ (i.e., prove that {ϕ | Γ `Σ ϕ} is a modal logic, that
{ϕ | Γ `Σ ϕ} contains Σ ∪ Γ, and that if modal logic Σ0 contains Σ ∪ Γ,
then {ϕ | Γ `Σ ϕ} ⊆ Σ0 ).
§4: Consistent and Maximal-Consistent Sets of Formulas
Some readers may have already encountered the idea that a set of formulas
Γ is consistent (relative to logic Σ) just in case there is no formula ϕ such
that both ϕ and ¬ϕ are deducible from Γ (in Σ). But the following
theorem shows this to be equivalent to saying that a set Γ is consistent
(relative to Σ) just in case the falsum is not derivable from Γ (in Σ)
(50) Theorem: Γ `Σ ⊥ iff there is a formula ϕ such that Γ `Σ ϕ & ¬ϕ.
Proof : By (45), given that ⊥ and ϕ & ¬ϕ are tautological consequences
of each other.
Remark : Given this equivalence, we adopt non-derivability of the falsum
as our definition of consistency:
(51) A set of formulas Γ is consistent Σ (in symbols: ConΣ (Γ) ) iff the
falsum (⊥) is not deducible from Γ in Σ. Formally,
55
ConΣ (Γ) =df Γ 6 `Σ ⊥.
We shall write CønΣ (Γ) whenever it is not the case that ConΣ (Γ).
Example 1 : Here is an example of a set Γ that is consistent relative to a
logic Σ but not consistent relative to logic Σ0 . A variant of the example in
(44) Remark 2 works well. Consider the set Γ3 = {(p → q), p, ¬q}.
Now relative to the logic PL, Γ3 is consistent. If our underlying logic
consists of the tautologies and the rule MP, then no contradictions can
be derivedPL from Γ3 . For example, the formula (p → q) → (p → q)
is not a tautology and so not a theorem of PL. So from the two members
of Γ3 , (p → q) and p, we can not derivePL q. Otherwise, Γ3 would
be inconsistentPL . So far, then, Γ3 looks like it is consistentPL . But we
have to show that we cannot derivePL the negation of any member of Γ3
from any combination of formulas in Γ3 . In fact, as the reader may verify,
there is no member of Γ3 having a negation that can be derivedPL from
some combination of members of Γ3 .
However, relative to the logic K, Γ3 is not consistent. From (p → q)
and p, one may deriveK q, as we saw in Remark 2 of (44). And since
¬q ∈ Γ3 , we know by (46.5) that Γ3 `K ¬q. Moreover, by (46.5),
Γ3 `K q. So, by (48.4), Γ3 `K q & ¬q, and so by (50), Γ3 `K ⊥.
Hence, by the definition of consistencyΣ , Γ3 is not consistentK .
Example 2 : Here is an example of a set that is inconsistent relative to
any logic containing the instances of the 5 schema: ♦ϕ → ♦ϕ. The
logic K5 is such a logic (it also contains instances of the K schema; K5
will be precisely defined in §4, but for now, the fact that it contains the
instances of the 5 schema is all that is important). Consider, then the set
Γ4 : {♦p, ♦p → q, ¬q}. We can prove that CønK5 (Γ4 ):
(a) First we show that Γ4 `K5 ♦p: Since ♦p ∈ Γ4 , we know by
(46.5) that Γ4 `K5 ♦p. Since K5 contains the instances of ♦ϕ → ♦ϕ,
♦p → ♦p ∈ K5 . So `K5 ♦p → ♦p. But by (46.2), Γ4 `K5 ♦p → ♦p.
So by (48.7), Γ4 `K5 ♦p.
(b) Next we show that Γ4 `K5 q: Since ♦p → q ∈ Γ4 , it follows that
Γ4 `K5 ♦p → q. But by (a), Γ4 `K5 ♦p. So again by (48.7), Γ4 `K5 q.
(c) Finally we show CønK5 (Γ4 ): Since ¬q ∈ Γ4 , we know that Γ4 `K5
¬q. But by (b), we also know that Γ4 `K5 q. So by (48.4), Γ4 `K5 q & ¬q.
Thus, by (50), Γ4 `K5 ⊥. Hence, CønK5 (Γ4 ).
(52) Theorem: The definition of consistent Σ has the following consequences:
56
.1) If ConΣ (Γ), then ⊥ 6∈ Γ and (ϕ & ¬ϕ) 6∈ Γ
.2) ConΣ (Γ) iff there is a ϕ such that Γ 6 `Σ ϕ
.3) If CønPL (Γ), then CønΣ (Γ)
.4) If CønΣ (Γ) and Σ0 is a Σ-logic, then CønΣ0 (Γ)
.5) If ConΣ (Γ) and ∆ ⊆ Γ, then ConΣ (∆)
.6) If CønΣ (∆) and ∆ ⊆ Γ, then CønΣ (Γ)
.7) ConΣ (Γ) iff for every finite subset ∆ of Γ, ConΣ (∆)
.8) Γ `Σ ϕ iff CønΣ (Γ ∪ {¬ϕ})
.9) ConΣ (Γ ∪ {ϕ}) iff Γ 6 `Σ ¬ϕ
.10) If ConΣ (Γ), then for any formula ϕ, either ConΣ (Γ ∪ {ϕ}) or
ConΣ (Γ ∪ {¬ϕ})
Remark : Explain what these signify.
(53) A set of formulas Γ is maximal (in symbols: Max(Γ) ) iff for every
ϕ, either ϕ ∈ Γ or ¬ϕ ∈ Γ. A set of formulas Γ is maximal-consistent Σ
(in symbols: MaxConΣ (Γ) ) iff both Max(Γ) and ConΣ (Γ).
Remark : Note that maximal-consistent sets are relativized to a logic.
Some sets are maximal-consistent relative to logic Σ but not maximalconsistent relative to logic Σ0 . For example.
(54) Theorem: Suppose MaxConΣ (Γ). Then:16
.1)
.2)
.3)
.4)
.5)
.6)
.7)
.8)
.9)
ϕ ∈ Γ iff Γ `Σ ϕ
Σ⊆Γ
⊥ 6∈ Γ
¬ϕ ∈ Γ iff ϕ 6∈ Γ
ϕ & ψ ∈ Γ iff both ϕ ∈ Γ and ψ ∈ Γ
ϕ ∨ ψ ∈ Γ iff either ϕ ∈ Γ or ψ ∈ Γ
ϕ → ψ ∈ Γ iff if ϕ ∈ Γ then ψ ∈ Γ
ϕ ↔ ψ ∈ Γ iff ϕ ∈ Γ if and only if ψ ∈ Γ
Γ is a Σ-logic
Remark : Note that in the right-left direction, (.1) asserts that maximalconsistent sets are deductively closed.
(55) Theorem (Lindenbaum’s Lemma): If ConΣ (Γ), then there is a MaxConΣ (∆)
such that Γ ⊆ ∆.
16 This
follows Chellas [1980], p.53.
57
Proof : Let ϕ0 , ϕ1 , ϕ2 , . . . be a listing of the sentences in ΛΩ . We define an
infinite sequence of sets of sentences ∆0 , ∆1 , ∆2 , . . . , and then the general
union ∆ of the members of the sequence:
∆0 = Γ
(
∆n+1 =
∆=
S
n≥0
∆n ∪ {ϕn }, if ∆n `Σ ϕn
∆n ∪ {¬ϕn }, otherwise
∆n
In other words, ∆0 is the set Γ, and ∆n+1 is constructed by adding ϕn
(the nth formula in the list) to ∆n if ϕn is derivableΣ from ∆n ; otherwise,
∆n+1 is constructed by adding ¬ϕn to ∆n . We show first that Γ ⊆ ∆
and then MaxConΣ (∆).
Lemma 1 : ∆n ⊆ ∆, for n ≥ 0.
Lemma 2 : Γ ⊆ ∆
To show MaxConΣ (∆), we prove first:
Lemma 3 : Max(∆)
Proof : We show, for arbitrary n, that either ϕn ∈ ∆ or ¬ϕn ∈ ∆.
Since either ∆n `Σ ϕn or ∆n 6 `Σ ϕn , we know ∆n+1 is equal either to
∆n ∪ {ϕn } or to ∆n ∪ {¬ϕn }. So either ϕn ∈ ∆n+1 or ¬ϕn ∈ ∆n+1 .
So by Lemma 1 , either ϕn ∈ ∆ or ¬ϕn ∈ ∆.
It remains only to show ConΣ (∆). Our strategy will be to show (a)
that all the ∆n s are consistent, and (b) that if ∆ were inconsistent, the
inconsistency would show up in one of the ∆n s, contradicting (a). We
show next that each of the ∆n s is consistentΣ :
Lemma 4 : ConΣ (∆n ), for n ≥ 0
Proof : By induction on the ∆n s.
(a) Base case: By hypothesis, ConΣ (Γ), and since Γ = ∆0 , we have
ConΣ (∆0 ).
(b) Inductive case: Either ∆n+1 = ∆n ∪{ϕn } or ∆n+1 = ∆n ∪{¬ϕn }
(but not both). (1) Suppose ∆n+1 = ∆n ∪{ϕn }. So by construction,
∆n `Σ ϕn . Hence, by (52.8), CønΣ (∆n ∪ {¬ϕn }). But by the
58
inductive hypothesis, ConΣ (∆n ). So by (52.10), ConΣ (∆n ∪ {ϕn }),
i.e., ConΣ (∆n+1 ). (2) Suppose ∆n+1 = ∆n ∪ {¬ϕn }. Then, by
construction, ∆n 6 `Σ ϕn . So, by (52.8), ConΣ (∆n ∪ {¬ϕn }), i.e.,
ConΣ (∆n+1 ).
If we are to argue that an inconsistency in ∆ would show up in one of
the ∆n s, it suffices to show, by (52.7), that every finite subset ∆0 of ∆
is a subset of one of the ∆n s, for by (52.6), the inconsistency would be
inherited by the ∆n of which ∆0 is a subset, contradicting the lemma we
just proved. We prove that every finite subset of ∆ is a subset of one of
the ∆n s as the last of the following three lemmas, the first two of which
are required for the proof:
Lemma 5 : ∆k ⊆ ∆n , for 0 ≤ k ≤ n.
Lemma 6 : if ϕk ∈ ∆, then ϕk ∈ ∆k+1 , for k ≥ 0
Lemma 7 : For every finite subset ∆0 of ∆, ∃n ≥ 0 such that ∆0 ⊆
∆n
Proof : Suppose ∆0 is a finite subset of ∆. Let ϕn be the sentence
in ∆0 with the largest index n. We show that ∆0 ⊆ ∆n+1 . Assume
ϕ ∈ ∆0 . Then ϕ = ϕk , for some k ≤ n. Since ∆0 ⊆ ∆, we know
that ϕ (= ϕk ) must be in ∆. So by Lemma 6 , ϕ ∈ ∆k+1 . But by
Lemma 5 , ∆k+1 ⊆ ∆n+1 . So ϕ ∈ ∆n+1 .
Finally, we argue:
Lemma 8 : ConΣ (∆)
Proof : Suppose CønΣ (∆). Then, by (52.7), there is a finite subset
∆0 of ∆ such that CønΣ (∆0 ). But by Lemma 7 , there exists an
n such that ∆0 ⊆ ∆n . So by (52.6), there exists an n such that
CønΣ (∆n ), contradicting Lemma 4 .
(56) Corollary 1 : Γ `Σ ϕ iff ϕ is an element of every MaxConΣ (∆) such
that Γ ⊆ ∆.
(57) Corollary 2 : `Σ ϕ iff for every MaxConΣ (∆), ϕ ∈ ∆
Proof : Let Γ in (56) be ∅ and use (46.1).
Exercise: Some modal logicians use the following, somewhat different
notion of a maximal-consistentΣ set: Γ is maximal-consistentΣ iff (a)
59
ConΣ (Γ) and (b) for every ϕ, if ConΣ (Γ∪{ϕ}), then ϕ ∈ Γ. The idea here
is that a maximal-consistent set is a consistent set Γ such that the addition
of one formula ϕ not already in Γ would result in an inconsistent set. Show
that this notion is equivalent to the notion that we have employed.
§5: Normal Logics
So far, we have been concerned with the properties of modal logics, possibly of the weakest kind. Modal logics are defined so as to capture, at
the very least, all of the propositionally correct forms of reasoning. As
such, they contain all the tautologies, and contain the tautological consequences of any combination of formulas they contain. We turn next to
a class of modal logics defined so as to capture the most basic, modally
correct, forms of reasoning. These logics contain not only the tautologies and propositionally correct consequences of formulas they contain,
but also both all the other valid formulas and all of the modally correct
consequences of any combination of formulas they contain. These are the
normal modal logics, the weakest of which is the logic K. The logic K
contains the tautologies, the instances of the axiom K, and is closed under
MP and the Rule of Necessitation. Since this will be the weakest normal
modal logic, we use ‘contains K and closed under RN’ as our definition of
a normal modal logic (see below). We show in the next chapter that K
is sound (its theorems are all valid) and complete (all valid formulas are
theorems of K).
(58) Recall that the rule RN is the sequence ϕ/ϕ. We say that a modal
logic Σ is normal iff every instance of the schema K is an element of Σ
and Σ is closed under RN.
(59) Theorem: A modal logic Σ is normal iff Σ is closed under the following rule RK: ϕ1 → . . . → ϕn → ψ / ϕ1 → . . . → ϕn → ψ.
Remark : This theorem shows that instead of defining normal modal logics as containing K and being closed under RN, we could have defined
them more simply as being closed under RK. From our work in (33) and
(34), we know that both of these rules preserve validity (in every class
of models) and truth in a model. However, in this case, we have chosen
to use the axiom K and rule RN because it may seem more intuitive to
readers accustomed to the conception of a logic as an axiom system and
because each piece of the notation KS1 . . . Sn , which we introduce below
60
to designate the normal logic containing the schemata K, S1 , . . . , Sn , is
linked to a schema that identifies the logic (so in the case when n = 0,
the normal logic K is identified by the axiom K).
(60) Theorem: If Σ is normal, then:
.1) `Σ ϕ ↔ ¬♦¬ϕ.
.2) `Σ ¬♦⊥
.3) `Σ ϕ, for any tautology ϕ
Remark : (.1) highlights the fact that propositional logic alone is not
sufficient for deriving the equivalence ϕ ↔ ¬♦¬ϕ from the definition
♦ϕ ↔ ¬¬ϕ. The added machinery that comes with normal logics is
essential for deriving the interdefinability of the and ♦. That is, the
interdefinability of the and ♦ requires modal inferences. This may
come as a surprise. Moreover, it is a consequence that if ♦ϕ is taken as a
primitive formula of the language rather than defined as ¬¬ϕ, then in
the definition of a normal logic we have to stipulate that normal logics,
in addition to containing K and being closed under RN, contain all the
instances of the valid schema ♦ϕ ↔ ¬¬ϕ, for otherwise we could not
preserve the interdefinability of the and ♦ in normal systems.
Exercise 1 : Prove that if Σ is a normal logic, then Σ contains the following
theorems and is closed under the following rules:17
♦¬ϕ ↔ ¬ϕ
ϕ → ψ/ϕ → ψ (‘RM’)
ϕ ↔ ψ/ϕ ↔ ψ (‘RE’)
ϕ → (ψ → χ)/ϕ → (ψ → χ) (‘RR’)
(ϕ & ψ) → (ϕ & ψ) (‘M’)
(ϕ & ψ) → (ϕ & ψ) (‘C’)
(ϕ & ψ) ↔ (ϕ & ψ) (‘R’)
(ϕ1 & . . . & ϕn ) → ϕ/(ϕ1 & . . . & ϕn ) → ϕ (‘RK&’)
ϕ → (ϕ1 ∨ . . . ∨ ϕn )/♦ϕ → (♦ϕ1 ∨ . . . ∨ ♦ϕn ) (‘RK∨’)
ϕ → ψ/♦ϕ → ♦ψ (‘RM♦’)
ϕ ↔ ψ/♦ϕ ↔ ♦ψ (‘RE♦’)
♦(ϕ ∨ ψ) ↔ (♦ϕ ∨ ♦ψ) (‘∨♦’)
♦(ϕ & ψ) → (♦ϕ & ♦ψ) (‘&♦’)
♦(ϕ1 & . . . & ϕn ) → (♦ϕ1 & . . . & ♦ϕn ) (‘&n ♦’)
17 Many
of the labels on these schemata and rules follow Chellas [1980], pp. 114–19.
61
Exercise 2 : Prove that (a) Σ is closed under RK iff Σ is closed under RR
and RN, (b) if Σ is normal, then Σ is closed under RK∨, and (c) if Σ is
closed under RK∨ and contains the instances of the schema ϕ ↔ ¬♦¬ϕ,
then Σ is normal.
Exercise 3 : (a) Show that (for any Σ) if MaxConΣ (Γ), then Γ is a modal
logic but not a necessarily a normal modal logic. (b) In particular, for
MaxConK (Γ), explain whether or not Γ must satisfy any of the conditions
that define normal logics.
Exercise 4 : Let ϕ[ψ 0 /ψ] be the result of replacing zero or more occurrences of ψ in ϕ by ψ 0 . Then every normal modal logic has the Rule of
Replacement: ψ ↔ ψ 0 /ϕ ↔ ϕ[ψ 0 /ψ].
(61) Theorem: The following are examples of normal modal logics:
.1) {ϕ | |=M ϕ}, for every model M (this is an example of a normal
modal logic that is typically not axiomatizable)
.2) {ϕ | |= ϕ}
.3) {ϕ | C |= ϕ}, for every class of models C
.4) {ϕ | F |= ϕ}, for every frame F
.5) {ϕ | CF |= ϕ}, for any class of frames CF .
.6) {ϕ | ϕ ∈ ΛΩ }
(62) We define the system K to be the smallest normal logic, i.e.,
T
K = {Σ| Σ is a normal modal logic}
Remark : This definition carves out the system K “from the top down.”
That is, we pare down all the sets which contain the tautologies, the
instances of the K axiom, and which are closed under MP and RN until
we reach the smallest one. As the smallest normal logic, K is a subset
of every normal logic. Thus, the only way a formula can qualify as a
theorem of K is by being a tautology, an instance of K, or by being the
conclusion by MP or RN of formulas in K. For suppose ϕ is a theorem
of K but neither a tautology, instance of K nor the conclusion by MP or
RN of formulas in K. Then the set K − {ϕ} would qualify as a normal
logic (since it still has all the tautologies, instances of K, and is closed
under MP and RN), yet would be a proper subset of K, contradicting
the fact that K is the smallest normal modal logic. Thus, K contains
nothing more than what it has to contain by meeting the definition of a
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normal logic. Consequently, to prove that all the theorems of K have a
certain property F , it suffices to prove (inductively) that the tautologies
and instances of K have F and that property F is preserved by the rules
of inference MP and RN.
Note, also, that our definition implies that K is a subset of each of
the examples of modal logics in (61).
(63) Theorem: K is axiomatized by the tautologies, the axiom schema
K, and the rules MP and RN.
Exercise: (a) Show that K is axiomatized by the tautologies and the rules
MP and RK; (b) Show that K is axiomatized by the axiom schema K and
the rules RPL and RN; (c) Show that K is axiomatized by the empty set
∅ of axioms and the rules RPL and RK.
(64) Following Lemmon [1977], we define the logic KS1 . . . Sn as the smallest normal logic containing (the instances of) the schemata S1 , . . . , Sn . Set
theoretically, we define:
T
KS1 . . . Sn =df {Σ | Σ is normal and S1 ∪ . . . ∪ Sn ⊆ Σ }
However, there are some names of normal modal logics which have already
become established in the literature and which do not follow this notation.
Here are some examples, identified in terms of our defined notation:
T = KT
B = KTB
S4 = KT4
S5 = KT5
K4.3 = K4L
S4.3 = KT4L
Remark : As the smallest normal logic containing S1 , . . . , Sn , KS1 . . . Sn is
a subset of every normal logic containing the schemata S1 , . . . , Sn . Thus,
the only way a formula can qualify as a theorem of KS1 . . . Sn is by being
a tautology, an instance of K, S1 , . . . , Sn , or by being the conclusion by
MP or RN of formulas already in KS1 . . . Sn . For otherwise, suppose ϕ is
a theorem of KS1 . . . Sn but neither a tautology, an instance of K, S1 , . . . ,
Sn , nor the conclusion by MP or RN of formulas in KS1 . . . Sn . Then the
set KS1 . . . Sn − {ϕ} would qualify as a normal logic containing K, S1 , . . . ,
Sn (since it still has all the tautologies, instances of K, S1 , . . . , Sn , and is
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closed under MP and RN), yet would be a proper subset of KS1 . . . Sn ,
contradicting the fact that KS1 . . . Sn is the smallest normal modal logic.
Consequently, to prove that the theorems of KS1 . . . Sn have property F ,
it suffices to prove (inductively) (a) that the tautologies and instances of
K, S1 , . . . , Sn have F and (b) that property F is preserved by the rules of
inference MP and RN.
(65) Theorem: KS1 . . . Sn is axiomatized by the tautologies, the axiom
schemata K, S1 , . . . , Sn , and the rules MP and RN.
§6: Normal Logics and Maximal-Consistent Sets
In this last section of Chapter 3, we work our way towards the proof
that maximal-consistent sets relative to normal logics have some rather
interesting properties. These properties play an important role in the
completeness theorems that we prove in the next chapter. In these proofs
of completeness, we develop a general method of constructing models for
consistent normal logics. The ‘worlds’ of the model for a logic Σ will be
the sets of sentences Γ that are maximal-consistent relative to Σ. In such
a model, it will turn out that a formula ϕ is true at the ‘world’ Γ just
in case ϕ is a member of Γ. Moreover, since a maximal-consistent set Γ
has as elements precisely the formulas derivable from it, when Γ plays
the role of a world, it will have the interesting property that the formulas
true at Γ will be precisely the ones derivable from Γ. In particular, when
a maximal-consistent Γ plays the role of a world in the models we shall
construct, any consequence of Γ is already ‘true’ at Γ.
When maximal-consistent sets play the role of worlds, an appropriate
accessibility relation R has to be defined. A proper definition of accessibility will have to ensure that when ϕ is a member of Γ, and Γ is
R-related to ∆, then ϕ will be a member of ∆. And, moreover, a proper
definition of the accessibility relation will have to ensure that whenever a
maximal-consistent set Γ is R-related to a maximal-consistent set ∆, then
any sentence ϕ that is a member of (i.e., true at) ∆ is such that ♦ϕ is a
member of (i.e., true at) Γ. In this section, then, we show that maximalconsistent sets of sentences in normal logics can be related to each other
in precisely these ways. We begin by proving some lemmas concerning
ordinary sets Γ (not necessarily maximal-consistent) in normal logics.
(66) Lemma: Suppose Σ is normal. Then if Γ `Σ ϕ, then {ψ| ψ ∈ Γ} `Σ
ϕ.
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Remark : This theorem shows that the deducibility relation in normal
logics behaves in a certain interesting way, namely, that whenever ϕ is
deducible from a set Γ (in normal logic Σ), then the necessitation of ϕ is
deducible (in Σ) from the set of the necessitations of the members of Γ.
(67) Lemma: Suppose Σ is normal. Then if {ψ| ψ ∈ Γ} `Σ ϕ, then
Γ `Σ ϕ
Remark : This lemma is an equivalent statement of the preceding one.
It says that if a formula ϕ is derivable (in a normal Σ) from the set
of sentences having necessitations in Γ, then the necessitation of ϕ is
derivable (in Σ) from Γ. It is instructive to understand why these two
lemmas are equivalent.
(68) Theorem: Suppose Σ is normal and that MaxConΣ (Γ). Then, ϕ ∈
Γ iff ϕ is a member of every MaxConΣ (∆) such that {ψ| ψ ∈ Γ} ⊆ ∆.
Remark : The real force of the present theorem is this. Think of Γ and
∆ as ‘worlds’, and think of the formulas that are members of these sets
as true at that world. Then, the clause {ψ| ψ ∈ Γ} ⊆ ∆ says that
ψ is ‘true at’ ∆ whenever ψ is ‘true at’ Γ. Intuitively, this means Γ
bears the accessibility relation R to ∆. Given this understanding, then,
this theorem is just an analogue, constructed out of syntactic entities, of
(6.5), i.e., of the conditions that must be satisfied if ϕ is to be true at
a world. For when we think of Γ as a world bearing R to ∆, then this
theorem just tells us that a formula ϕ is true at Γ iff ϕ is true at every
R-related world ∆.
Exercise: Find a proof of this theorem in the right-left direction using
(66) instead of (67).
(69) Lemma: Suppose Σ is normal, MaxConΣ (Γ), and MaxConΣ (∆).
Then, [for every ϕ, if ϕ ∈ Γ, then ϕ ∈ ∆] iff [for every ϕ, if ϕ ∈ ∆, then
♦ϕ ∈ Γ]
Remark : Here is a slightly more efficient way to state the present lemma:
{ϕ | ϕ ∈ Γ} ⊆ ∆ iff {♦ϕ | ϕ ∈ ∆} ⊆ Γ. To see what this says, think of
Γ and ∆ as ‘worlds’ and membership in as ‘truth at’. Then, the lemma
says that [Γ and ∆ are R-related in the sense that ϕ is true in (i.e., a
member of) ∆ whenever ϕ is true in (i.e., a member of) Γ] iff [Γ and ∆
are also R-related in the sense that ♦ϕ is true in Γ whenever ϕ is true in
∆]. Since normal logics are modally well-behaved in the sense that they
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contain the usual biconditionals interdefining the and ♦, whenever two
maximal consistent sets instantiate the R relation with respect to one
modality, they automatically instantiate the R relation with respect to
the other modality.
(70) Theorem: Suppose that Σ is normal and that MaxConΣ (Γ). Then
♦ϕ ∈ Γ iff there is a MaxConΣ (∆) such that both (a) {♦ψ| ψ ∈ ∆} ⊆ Γ
and (b) ϕ ∈ ∆.
Remark : The present theorem does for the ♦ what (68) does for the
. Think of Γ and ∆ as worlds. Suppose further that the accessibility
relation R holds when formulas true at ∆ are possibly true at Γ. Then
this theorem is just an analogue, constructed out of syntactic entities, of
Remark 2 in (6.5), i.e., of the conditions that must be satisfied if ♦ϕ is
to be true at a world. For when we think of Γ as a world bearing R to
∆, then this theorem just tells us that a formula ♦ϕ is true at (i.e., a
member of) Γ iff ϕ is true at some R-related world ∆.
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Chapter Five:
Soundness and Completeness
In this chapter, we assemble the results of the previous two chapters
so that we may show that certain normal modal logics are sound and
complete with respect to certain classes of models and classes of frames.
§1: Soundness
(71) A logic Σ is sound with respect to a class of models C iff every
theorem of Σ is valid with respect to C. That is,
Σ is sound with respect to C =df for every ϕ, if `Σ ϕ, then C |= ϕ
Remark : In what follows we prove, for Σ = K and Σ = KS1 . . . Sn , that Σ
is sound with respect to a certain class C of models. Our argument shall
consist of two claims: (a) that the tautologies and schemata identifying
Σ are valid with respect to C, and (b) that the rules of inference MP and
RN preserve validity with respect to C. There are two different reasons
why this argument establishes that Σ is sound with respect to C. We
discuss them in turn.
(.1) Recall that the logic K is the smallest normal logic. In the Remark
in (63), we established that the only way a formula can qualify as a
theorem of K is by being a tautology, an instance of K, or by being the
conclusion by MP or RN of formulas in K. Consequently, to prove that
all the theorems of K have a certain property F , it suffices to prove that
the tautologies and instances of the schema K have F and that property
F is preserved by the rules of inference MP and RN. In particular, if we
want to show that the theorems of the logic K are all valid with respect
to the class of all models, then we show that the tautologies and instances
of the schema K are valid with respect to this class, and that MP and RN
preserve validity with respect to this class.
Similar remarks apply to KS1 . . . Sn . In the Remark in (65), we established that the only way a formula can qualify as a theorem of KS1 . . . Sn
is by being a tautology, an instance of K, S1 , . . . , Sn , or by being the conclusion by MP or RN of formulas already in KS1 . . . Sn . So to prove that
the theorems of KS1 . . . Sn have property F , it suffices to prove (inductively) (a) that the tautologies and instances of K, S1 , . . . , Sn have F and
(b) that property F is preserved by the rules of inference MP and RN.
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In particular, if we want to show that the theorems of KS1 . . . Sn are all
valid with respect to a class of models C, then we show that the tautologies and instances of the schemata K, S1 , . . . , Sn are valid with respect to
C, and that MP and RN preserve validity with respect to C.
(.2) The other reason why this two-part argument for the soundness
of K and KS1 . . . Sn is sufficient derives from the fact that these logics
are axiomatizable. Consider the following lemma:
Lemma: If (i) Σ is axiomatized by the set Γ and the rules R1 , R2 , . . . ,
and (ii) the members of Γ are valid with respect to C, and (iii) the
rules preserve validity with respect to C, then Σ is sound with
respect to C.
Thus, since we know in particular that K is axiomatized by the tautologies, the schema K, and the rules MP and RN, to establish the soundness
of Σ with respect to the class of all models, we need only argue (a) that
the tautologies and K are valid with respect to the class of all models
and (b) that MP and RN preserve validity with respect to the class of
all models. Similarly for the logic KS1 . . . Sn . Given that KS1 . . . Sn is
axiomatized by the tautologies, K, S1 , . . . , Sn , and the rules MP and RN,
we need only argue (a) that the tautologies and K, S1 , . . . , Sn are valid
with respect to the class C of models, and (b) the rules MP and RN
preserve validity with respect to C, to show that the logic KS1 . . . Sn is
sound with respect to C.
(72) Theorem: K is sound with respect to the class of all models.
Proof : Given the lengthy Remark in (71), this follows from the facts that
(a) the tautologies and instances of K are valid, by (16) and (17), and so
valid with respect to the class of all models, and (b) that MP and RN
preserve validity with respect to any class of models, by (30) and (33).
Remark : There is now in the literature an even more ingenious proof of
the soundness of K. The argument requires little commentary, since it
is so simple. Assume `K ϕ, i.e., ϕ ∈ K (to show |= ϕ). Since K is the
smallest normal modal logic, K is a subset of every normal modal logic.
In particular, by (61), {ψ | |= ψ} is a normal modal logic. So K is a
subset of this set. Hence ϕ ∈ {ψ | |= ψ}, i.e., |= ϕ. Since ϕ is valid, it is
valid with respect to the class of all models.
(73) Theorem: Let CP designate the class of all models M such that
RM has property P. Suppose that S1 , . . . , Sn are schemata which are
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valid, respectively, in the classes of standard models CP1 , . . . , CPn . Let
CP1 . . . Pn designate the class of models CP1 ∩. . .∩CPn . Then the modal
logic KS1 . . . Sn is sound with respect to CP1 . . . Pn ; i.e., if `KS1 ...Sn ϕ,
then CP1 . . . Pn |= ϕ.
Proof : Given the lengthy Remark in (71), it suffices to argue as follows:
(a) If ϕ is a tautology or an instance of K, ϕ is valid, and so valid with
respect to CP1 . . . Pn . If ϕ is an instance of Si (1 ≤ i ≤ n), then by
hypothesis, ϕ is valid with respect to CPi . But if so, then ϕ is valid with
respect to the class CP1 ∩ . . . ∩ CPi ∩ . . . ∩ CPn , since this is a subset of
CPi . So, for each i, the instances of Si are valid with respect to the class
CP1 . . . Pn . And so, in general, the instances of the axioms K, S1 , . . . , Sn
are valid with respect to the class of models CP1 . . . Pn . (b) MP and RN
both preserve validity with respect to any class of models, and so preserve
validity with respect to CP1 . . . Pn .
Remark 1 : Now reconsider the facts proved in (23). These facts establish
that certain schemata are valid with respect to certain classes of models.
So, for example, we know that T is valid with respect to the class of
reflexive models, and 5 is valid with respect to the class of euclidean
models. We have as an instance of the present theorem, therefore, that
the logic KT5 is sound with respect to the class of all reflexive, euclidean
models, i.e., if `KT5 ϕ, then C-refl,eucl |= ϕ. Similarly, for any other
combination of the schemata in (23).
Remark 2 : Again, we may argue that KS1 . . . Sn is sound with respect to
C in a somewhat more ingenious manner. By hypothesis, the instances of
Si are valid with respect to the class of models CPi (1 ≤ i ≤ n). So the
instances of Si are valid with respect to the class of models CP1 . . . Pn ,
since this is a subset of CPi . Consequently, the instances of S1 , . . . ,
Sn are elements of {ψ | CP1 . . . Pn |= ψ}. But by (61.4), this set is a
normal logic, and so it is a normal logic containing (the instances of)
S1 ,. . . ,Sn . But by definition, KS1 . . . Sn is the smallest logic containing
S1 , . . . , Sn . So KS1 . . . Sn must be a subset of {ψ | CP1 . . . Pn |= ψ}. That
is, if ϕ ∈ KS1 . . . Sn , then ϕ ∈ {ψ | CP1 . . . Pn |= ψ}. In other words, if
`KS1 ...Sn ϕ, then CP1 . . . Pn |= ϕ. Thus, KS1 . . . Sn is sound with respect
to CP1 . . . Pn .
Exercise: Reconsider the Remark 1 in (28) in light of the present proof
of soundness for various systems. Show that the schemata B, 4, and 5
are not theorems of KT . Show that the schema 4 is not a theorem of the
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logic KB . What other facts can you prove from the results in (28) given
our soundness results?
(74) Let us say that Σ is sound with respect to a class of frames CF iff
every theorem of Σ is valid with respect to CF . That is,
Σ is sound with respect to CF =df for every ϕ, if `Σ ϕ, then CF |= ϕ
Remark : Note that for a logic Σ to be sound with respect to a class of
frames, the theorems of Σ must be true in every model based on any frame
in the class.
(75) Theorem: K is sound with respect to the class of all frames.
Proof : Use reasoning analogous to that used in the Remark in (72).
(76) Theorem: Let the class of P-frames (in symbols: CF P) be the class
of all frames F in which RF has property P. Then KS1 . . . Sn is sound
with respect to the class of all P1 . . . Pn -frames, i.e., if `KS1 ...Sn ϕ, then
CF P1 . . . Pn |= ϕ.
Proof : Use reasoning that generalizes on that used in the previous theorem.
§2: Completeness
(77) A logic Σ is complete with respect to a class C of models iff every
formula valid with respect to C is a theorem of Σ. Formally:
Σ is complete with respect to C =df for every ϕ, if C |= ϕ, then
`Σ ϕ
For example, to say that the logic K4 is complete with respect to the
class of transitive models is to say that every formula valid in the class
of transitive models is a theorem of K4 ; i.e., for every ϕ, if C-trans |= ϕ,
then `K4 ϕ.
Extended Remark : The favored way of establishing that Σ is complete
relative to C is by proving the ‘contrapositive’ of the definition, that is,
by proving that if ϕ is not a theorem of Σ, then ϕ is not valid with respect
to C. This is a helpful way of picturing and understanding the definition.
Somewhat more formally, the ‘contrapositive’ amounts to:
(A) For every ϕ, if 6 `Σ ϕ, then ∃M ∈ C such that 6|=M ϕ.
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So to prove completeness, one might look for a general way of constructing, for an arbitrary non-theorem ϕ of Σ, a model M ∈ C which falsifies
ϕ.
In fact, however, it turns out that in developing proofs that certain
consistent normal logics are complete, logicians have discovered a general
way of proving something even stronger. They have discovered a way of
constructing a unique model M ∈ C which falsifies every non-theorem of
Σ! This construction technique yields a very general method for proving:
(B) ∃M ∈ C such that for every ϕ, if 6 `Σ ϕ, then 6|=M ϕ
The model M which falsifies all the non-theorems of Σ is called the canonical model of Σ (in symbols: MΣ ). For example, from our soundness results, we know that the instances of the schema 4 (= ϕ → ϕ) are not
theorems of KT (since 4 is not valid with respect to the class of reflexive
frames). So the canonical model for KT , MKT , will contain worlds that
falsify the instances of the 4 schema. These will be worlds at which ϕ
is true, but at which ϕ is not true.
Note that (B) entails (A), but not vice versa. Clearly, if there is a
model which falsifies every non-theorem of Σ, then for any non-theorem of
Σ, there is a model which falsifies it. Thus, by constructing the canonical
model M for Σ, showing that M ∈ C and that M falsifies every nontheorem of Σ, we prove (A), which, by the reasoning in the previous
paragraph, establishes that Σ is complete with respect to C. This, then,
is the general strategy we shall pursue in proving completeness.
The canonical model MΣ for a logic Σ is able to do its job of falsifying
every non-theorem because it has some very special features. The most
important feature that MΣ has is that its worlds are just all the MaxConΣ
sets. By defining the accessibility relation and valuation function of MΣ
in the right way, we shall be able to show that truth at a world, i.e., at a
MaxConΣ (Γ), in the canonical model just is membership in Γ. This leaves
MΣ with another very special feature. Given Corollary 2 to Lindenbaum’s
Lemma, we know that the formulas true in all the MaxConΣ sets are
precisely the theorems of Σ. So, given our remarks about defining truth,
the theorems of Σ will be true in MΣ , since they are true at (i.e., members
of) every world (i.e., MaxConΣ set). Thus, MΣ will have the special
feature of determining Σ in the sense that all and only the theorems of Σ
are true in MΣ . Formally:
Σ
MΣ determines Σ =df |=M ϕ if and only if `Σ ϕ, for every ϕ
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Clearly, if MΣ has this feature, then every non-theorem of Σ is false at
some world in MΣ . So if we can prove that MΣ determines Σ and that MΣ
is in the class of models C, then a fortiori , we have established (B). So the
actual proof of completeness divides up into two parts: (a) a proof that
MΣ determines Σ, and (b) a proof that MΣ ∈ C. Part (a) is completely
general, and is proved only once, for arbitrary MΣ and Σ. Part (b) is
proved individually for each particular logic Σ and class C. For example,
to show that K4 is complete with respect to the class of transitive models,
we show: (a) that the canonical model MK4 determines K4 , and (b) that
MK4 is an element of the class of transitive models, i.e., that RK4 (i.e,
the accessibility relation of MK4 ) is transitive. Part (a) of the proof is
an automatic consequence of the very general result that every canonical
model determines its corresponding logic. Part (b) of the proof, however,
requires that we argue that the particular accessibility relation RK4 of
the canonical model is transitive. This shows that MK is in the class of
transitive models C-trans. This is a required step if we are to show that
K4 is complete with respect to C-trans. For other logics Σ, we have to
argue in part (b) of the completeness proof that RΣ (i.e., the accessibility
relation of MΣ ) has the relevant property P. This shows that MΣ is in the
class of all P-models CP. This step is needed if we are to show that Σ is
complete with respect to CP. For example, for part (b) of the proof that
K5 is complete with respect to the class of euclidean models, we have to
argue that the accessibility relation of RK5 is euclidean. This shows that
MK5 is in the class of euclidean models C-eucl, which given (a), shows
that K5 is complete with respect to C-eucl. And so forth.
The proofs of part (b) for the various logics are based on another
special feature of canonical models: the proof-theoretic properties of logic
Σ have an effect on the maximal-consistentΣ sets that serve as the ‘worlds’
of MΣ . To see how, note for example, that the ‘worlds’ of the canonical
model MK4 for the logic K4 will be the sets of sentences that are maximalconsistent relative to K4 . By (57), it follows that since ϕ → ϕ is
a theorem of K4 , each instance of this schema is a member of all of
the maximal-consistentK4 sets of sentences. So all of the worlds in the
canonical model MK4 will contain all of the instances of the 4 schema.
This fact affects the accessibility relation of MK4 , which is defined so as to
preserve the theorems concerning normal logics and maximal consistent
sets which were proved in §6 of Chapter 4. It can be shown that if
all the ‘worlds’ in MK4 contain the instances of the 4 schema, then the
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accessibility relation RK4 of the canonical model will have to be transitive
(this is proved in (82)). Similarly for other logics and canonical models.
For example, it follows from the fact that the instances of the 5 schema
are members of all the maximal-consistentK5 sets that the accessibility
relation of MK5 is euclidean. This, then, is how we prove part (b) of
the completeness proofs, i.e., that MΣ is in the relevant class of models
C. We prove it by showing that RΣ has the relevant property, and the
fact that it does derives from the effect the proof-theoretic properties of
the logic Σ have on maximal-consistent sets relative to Σ. The resulting
group of maximal-consistent sets, in turn, have an effect on the properties
of RΣ .
(78) The canonical model of a consistent normal logic Σ is the model MΣ
(= hWΣ , RΣ , VΣ i) that satisfies the following conditions:
.1) WΣ = {Γ | MaxConΣ (Γ)}
.2) RΣ ww0 iff {ϕ | ϕ ∈ w} ⊆ w0
.3) VΣ (p) = {w ∈ WΣ | p ∈ w}
Note that if Σ is not consistent, then there are no MaxConΣ sets and so
MΣ does not exist.
Remark 1 : Clause (.1) tells us that in the canonical model MΣ , the worlds
in WΣ are precisely the sets of formulas which are maximal-consistent
relative to Σ. Clause (.2) requires that the accessibility relation RΣ holds
between w and w0 just in case ϕ ∈ w0 whenever ϕ ∈ w. Note that this
definition of R allows us to appeal to the theorems concerning normal
logics and maximal-consistent sets in §6 of Chapter 4. So by (69), clause
(.2) is equivalent to saying that ♦ϕ ∈ w whenever ϕ ∈ w0 . Finally, clause
(.3) says that the valuation function VΣ is defined so that an atomic
formula p is true at all the worlds of which it is an element. This will
prove to be the basis for arguing that, for all w, ϕ is true at w in MΣ iff
ϕ ∈ w. And given (57), this in turn will be the basis for arguing that ϕ
is true in MΣ iff ϕ is a theorem of Σ (i.e., that MΣ determines Σ).
Remark 2 : Here is an intuitive picture of the facts proved in what follows.
Note that by (57), if ϕ is not a theorem of Σ, then there is a maximalconsistentΣ set that fails to contain ϕ. So by (54.4), there is a maximalconsistentΣ set that contains ¬ϕ. So, since our maximal-consistentΣ sets
serve as the worlds in MΣ , the negation of each non-theorem of Σ gets
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embedded in some world in WΣ . Intuitively, this means that each nontheorem of Σ will be false in some world. Thus, MΣ is rich enough
to contain a falsifying world for each non-theorem. For example, the
negation of each instance of the T schema will get embedded in some
maximal-consistentK set in WK (since the instances of the T schema are
non-theorems of K). Consider, for example, the instance p → p. Our
definitions require that there be a world w in WK containing ¬(p →
p), and therefore, by the properties of maximal-consistency, containing
p & ¬p, p, and ¬p (any such ‘world’ is consistent relative to K).
Moreover, by the definition of the accessibility relation, any world w0
in WK such that Rww0 will contain p, since w contains p. In this
manner, the canonical model MΣ will contain a world that falsifies each
non-theorem of Σ.
We turn, then, to the theorems that prove that this picture is an
accurate one.
(79) Lemma: Let MΣ be the canonical model for Σ. Then, for every
Σ
w ∈ WΣ , |=M
w ϕ iff ϕ ∈ w.
Σ
(80) Theorem: |=M ϕ iff `Σ ϕ, for every consistent, normal modal logic
Σ.
Remark 1 : It is interesting to note that MΣ will typically contain isolated
groups of worlds, i.e., groups γ and δ of maximal-consistent sets such
that the members of γ may be R-related in some way to each other, and
the members of δ may be R-related in some way to each other, but no
maximal-consistent set in γ is R-related to any maximal-consistent set in
δ, and vice versa. Here is an argument that shows why this must occur.
Note that in KT5 (i.e., S5 ) for example, the formula ¬p is not a theorem
S5
(why?). So there is a world w in WS5 such that p ∈ w (i.e., |=M
p).
w
Consequently, ♦p is true in any world that accesses w. Let γ be the group
of worlds that access w. Not every world can be in γ, for otherwise every
world would contain ♦p and so ♦p would have to be a theorem of S5 . But
♦p is not a theorem of S5 (why?). So there must be some world w0 that
contains ¬♦p, and thus, ¬p. But for ¬p to be true at w0 , ¬p must
be true at worlds that w0 can access. Call the group of worlds that w0
can access δ. Claim: There can be no relationship between any members
of γ and δ. Argument: Let w1 be an arbitrary member of γ and w2 be
an arbitrary member of δ. Suppose that Rw1 w2 . Then by hypothesis
Rw1 w. But MS5 is reflexive, euclidean, and so by the euclidean property,
74
Rw2 w. But by (29.3), reflexive euclidean models are transitive. So by
transitivity, and the fact that both Rw0 w2 (by hypothesis) and Rw2 w,
it follows that Rw0 w. Since ¬p is true at w0 , it follows that ¬p is true
at w, which contradicts our initial assumption that p ∈ w, i.e., that p is
true in w. Analogous reasoning establishes that ¬Rw2 w1 .
Remark 2 : We’ve now established in general that the canonical model
MΣ determines Σ, for any consistent, normal logic Σ. Recall that to
prove a logic Σ complete with respect to a class C of models, our strategy
is show (a) that MΣ determines Σ, and (b) that MΣ ∈ C. Together, (a)
and (b) establish that ∃M ∈ C such that if 6 `Σ ϕ then C 6|= ϕ, which in
turn establishes that if C |= ϕ then `Σ ϕ (i.e., that Σ is complete with
respect to C). It remains therefore to show part (b) for a each logic Σ.
That is, we need to show that RΣ has the property that qualifies M as
a member of the relevant class C. In the case of the logic K, we need to
show very little, for K is complete with respect to the class of all models.
For K at least, we need only show that ∃M in the class of all models such
that M determines K. This is immediate, since the canonical model MK
is in the class of all models.
(81) Theorem: K is complete with respect to the class C of all models.
K
Proof : By (80), |=M ϕ iff `K ϕ. Hence, ∃M ∈ C such that if 6`K ϕ,
then 6|=M ϕ. A fortiori , if 6`K ϕ, then ∃M ∈ C such that 6|=M ϕ. Thus, if
6`K ϕ, then ϕ is not valid with respect to C, and so K is complete with
respect to the class of all models.
Remark : We now turn to the penultimate step in the proofs of completeness. In virtue of our earlier remarks, we know that to prove a logic Σ is
complete with respect to a class of models C, it is now sufficient to show
that the proper canonical model MΣ is an element of C. To do this, we
show that if a logic Σ contains the instances of the axiom schema Si , for
the schemata in (23), then the accessibility relation RΣ of the canonical
model MΣ satisfies the corresponding property Pi .
(82) Lemma: Let Pi and Si be as in (23). Let Σ be normal. Then if Σ
contains Si , RΣ satisfies Pi .
(83) Theorem: KS1 . . . Sn is complete with respect to the class CP1 . . . Pn
of models. That is, if CP1 . . . Pn |= ϕ, then `KS1 ...Sn ϕ.
Remark : Here are two instances of this theorem: (a) The logic KT4 is
complete with respect to the class of reflexive, transitive models, and
75
(b) the logic KTB4 is complete with respect to the class of reflexive,
symmetric, and transitive models.
(84) Exercise: Reconsider the facts established in (29) and the Remark
about those facts. Prove that KT is an extension of KD. Prove that KB4
is the same logic as KB5 . Prove that KTB4 = KT5 = KDB4 = KDB5 .
What other facts about modal logics can be established on the basis of
(29) in light of the completeness results?
76
Chapter Six: Basic Quantified Modal Logic
§1: The Simplest Quantified Modal Logic
(85) Vocabulary: The primitive non-logical vocabulary of our language is
a set C of constants consisting of object constants and relation constants.
Specifically, C = OC ∪ RC, where:
.1) OC is the set of object constants: {a1 , a2 , . . .}
.2) RC is the set of relation constants: {P1n , P2n , . . .} (n ≥ 0)
We use a, b, . . . as typical members of OC, and P n , Qn , Rn . . . as typical
members of RC. In addition to this non-logical vocabulary, we have the
following logical vocabulary:
.3) The set V of (object) variables: {x1 , x2 , . . .}
(We use x, y, z, . . . as typical members of V.)
.4) The truth functional connectives ¬ and →, the quantifier ∀, and the
modal operator . For languages with identity, we add the identity
sign =.
(86) Terms: We define the set T of terms to be the set OC ∪ V.
(87) Formulas: The set Fml (C) of formulas based on the non-logical
vocabulary C is the smallest set satisfying the following conditions:
.1) If P n ∈ RC, and τ1 , . . . , τn ∈ T , then P n τ1 . . . τn ∈ Fml (C) (n ≥ 0).
.2) If ϕ, ψ ∈ Fml (C), and x ∈ V, then (¬ϕ), (ϕ → ψ), (∀xϕ), and
(ϕ) ∈ Fml (C).
For languages with identity, we add:
.3) If τ, τ 0 ∈ T , then (τ = τ 0 ) ∈ Fml (C)
(88) Models: A model M for the non-logical vocabulary C is any quadruple hW, D, R, Vi satisfying the following conditions:
.1) W is a non-empty set of worlds,
.2) D is a non-empty domain of objects,
.3) R is a binary accessibility relation on W, i.e., R ⊆ W × W,
77
.4) V is a valuation function that has the set C as its domain and meets
the following conditions:
a) If a ∈ OC, V(a) ∈ D,
b) If P n ∈ RC and n = 0, V(P n ) ∈ P(W),
c) If P n ∈ RC and n ≥ 1, V(P n ) ∈ {g | g : W → P(Dn )}.
(89) M-Assignments to variables: An M-assignment to the variables is
any function f such that f : V → D. If f and f 0 are both assignments, we
x
write f 0 = f whenever f 0 is an assignment identical to f except perhaps
for what it assigns to x.
(90) Denotations of terms: The denotation of term τ with respect to
model M and M-assignment f (in symbols: dM,f (τ )) is defined as follows:
.1) If τ ∈ OC, dM,f (τ ) = V(τ )
.2) If τ ∈ V, then dM,f (τ ) = f (τ )
(91) Satisfaction: We define: f satisfies M formula ϕ with respect to world
w as follows:
.1) (n ≥ 1) f satisfiesM P n τ1 . . . τn wrt w iff
hdM,f (τ1 ), . . . , dM,f (τn )i ∈ [V(P n )](w)
(n = 0) f satisfiesM P 0 wrt w iff w ∈ V(P 0 )
.2) f satisfiesM ¬ψ wrt w iff f fails to satisfyM ψ wrt w
.3) f satisfiesM ψ → χ wrt w iff
either f fails to satisfyM wrt w or f satisfiesM χ wrt w
.4) f satisfiesM ∀xψ wrt w iff
x
for every f 0 , if f 0 = f , then f 0 satisfiesM ψ wrt w
.5) f satisfiesM ψ wrt w iff
for every w0 , if Rww0 , then f satisfiesM ψ wrt w0 .
For languages with identity, we add:
.6) f satisfiesM τ = τ 0 wrt w iff dM,f (τ ) = dM,f (τ 0 )
78
(92) Truth at a World : We say ϕ is true M at world w (in symbols: |=M
w ϕ)
iff every assignment f satisfiesM ϕ with respect to w.
(93) Truth: We say that ϕ is true M (in symbols: |=M ϕ) iff for every
world w, |=M
w ϕ (i.e., iff for every world w, ϕ is trueM at w)
(94) Validity: We say ϕ is valid (in symbols: |= ϕ) iff for every model M,
|=M ϕ (i.e., iff for every M, ϕ is trueM ).
(95) Logical Consequence: ϕ is a logical consequence of a set Γ with
respect to a class of models C (in symbols: Γ |=C ϕ) iff ∀M ∈ C, ∀w ∈ M,
M
if |=M
w Γ, then |=w ϕ.
(96) Logic: A set Γ is a quantified modal logic iff
.1) Γ is closed under RPL
.2) Γ contains the instances of the following schemata:
a) ∀xϕ → ϕτx , provided τ is substitutable for x in ϕ.18
b) ∀x(ϕ → ψ) → (ϕ → ∀xψ), where x is any variable not free in
ϕ
.3) Γ is closed under the Rule of Generalization: ϕ/∀xϕ
If the language has identity, then we say that Γ is a quantified modal logic
with identity in case Γ satisfies the above definition with the following two
additions to clause (.2):
c) x = x, for any variable x
d) x = y → (ϕ(x, x) ↔ ϕ(x, y)), where ϕ(x, y) is the result
of replacing some, but not necessarily all, occurrences of
x by y in ϕ(x, x).
A set Γ is a normal quantified modal logic (with identity) if in addition to
being a quantified modal logic (with identity), Γ contains all the instances
of the schema K, all the instances of the Barcan formula (= ∀xϕ →
∀xϕ), and is closed under RN.
18 The symbol ϕτ stands for the result of substituting the term τ for free occurrences
x
of the variable x everywhere in ϕ. τ is substitutable for x in ϕ provided no variable y
in τ is captured by a quantifier ∀y in ϕτx .
79
§2: Kripke’s Semantical Considerations on Modal Logic
(97) Kripke’s System:
See Kripke’s paper, ‘Semantical Considerations on Modal Logic’,
Acta Philosophica Fennica 16 (1963): 83-94
(98) Formulas Valid in the Simplest QML That Are Invalid in Kripke’s
System:
∀xP x → P y
∀xP x → ∃xP x
∀xP x → ∀xP x
∀xP x → ∀xP x
∃y y = x
(F x → F x)
(99) Reasons Philosophers Cite for Excluding These Formulas:
(100) Techniques for Excluding These Formulas:
Defining the Existence Predicate as Quantification
Variable Domains
Free Logic
Generality Interpretation — Defining Validity only for Closed Formulas
(101) Reasons for Preferring the Simpler Quantified Modal Logic:
See the paper, ‘In Defense of the Simplest Quantified Modal Logic”,
coauthored by Bernard Linsky and Edward N. Zalta, in Philosophical Perspectives (8): Philosophy of Logic and Language, J. Tomberlin (ed.), Atascadero: Ridgeview, 1994
80
§3: Modal Logic and a Distinguished Actual World
(102) Reasons for Formulating Modal Logic with a Distinguished Actual
World.
See the paper, ‘Logical and Analytic Truths That Are Not Necessary’, by Edward N. Zalta, in the Journal of Philosophy, 85/2
(February 1988): 57–74
81
Appendix: Proofs of Theorems and Exercises
Chapter Three
Proof of Exercise 2 in (7):
(a) Assume |=M ϕ → ψ. So for every w, |=M
w ϕ → ψ. Assume now
0
M
that |=M ϕ. So for every w, |=M
w ϕ. Pick an arbitrary w . So |=w0 ϕ → ψ
M
M
0
and |=w0 ϕ. Hence, by (6.4), |=w0 ψ. So, since w was arbitrary, for every
M
world w, |=M
w ψ, i.e., |= ψ. So, by our conditional proof, it follows from
M
M
|= ϕ → ψ that if |= ϕ then |=M ψ.
(b) Consider the following M: let W = {w1 , w2 }; R be empty; and
V(p) = {w1 } and VM (q) = { }. Since 6|=M
w2 p, we know that not every
M
world w is such that |=M
w p. Hence, 6|= p, and thus by antecedent failure,
M
M
if |=M p then |=M q. However, |=M
w1 p and 6|=w1 q, and 6|=w1 p → q. So not
M
every w is such that |=M
w p → q. Thus, 6|= p → q. This means we have a
M
model in which the conditional if |= ϕ then |=M ψ holds, but for which
6|=M ϕ → ψ, demonstrating that the conditional, if |=M ϕ then |=M ψ,
does not imply |=M ϕ → ψ.
Proof of (12): By induction on ΛΩ (though the induction proceeds by
considering, in the base case, the quasi-atomic formulas p∗ , and then, in
the inductive cases, the formulas ⊥, ¬ψ, and ψ → χ). Base case: Suppose
that ϕ = p∗ . If f is based on f ∗ , then for every q ∈ Ω∗ , f (q ∗ ) = f ∗ (q ∗ ).
So f (p∗ ) = f ∗ (p∗ ). Similarly, f 0 (p∗ ) = f ∗ (p∗ ). So f (p∗ ) = f 0 (∗ ); i.e.,
f (ϕ) = f 0 (ϕ).
Induction cases: (1) Suppose that ϕ = ⊥. Then, by the definition of
subformula, ⊥ is a subformula of ϕ, since every formula is a subformula
of itself. So by hypothesis, f (⊥) = f 0 (⊥), and hence, f (ϕ) = f 0 (ϕ).
(2) Suppose ϕ = ¬ψ. We may assume, as an inductive hypothesis, that
the theorem holds for ψ, i.e., we may assume for the inductive hypothesis
that f (ψ) = f 0 (ψ). Exercise: Complete the proof that f (ϕ) = f 0 (ϕ).
(3) Suppose that ϕ = ψ → χ. We may assume as inductive hypotheses
both that f (ψ) = f 0 (ψ) and that f (χ) = f 0 (χ). Exercise: Finish the proof
that f (ϕ) = f 0 (ϕ).
Proof of Exercise in (13): By induction on ΛΩ . Base case: Suppose
that ϕ = p∗ . By the definition of TFR-subformula, every formula is a
subformula of itself. So p∗ is a subformula of ϕ. But p∗ is quasi-atomic,
and so by the hypothesis of the theorem (which gives us that f and f 0
82
agree on the TFR-subformulas in ϕ), we know that f (p∗ ) = f 0 (p∗ ). So
f (ϕ) = f 0 (ϕ).
Inductive cases: (1) Suppose that ϕ = ⊥. Then, again by the definition of TFR-subformula, ⊥ is a TFR-subformula of ϕ. So by hypothesis,
f (⊥) = f 0 (⊥), and hence, f (ϕ) = f 0 (ϕ).
(2) ϕ = ¬ψ. (The proof of this is just like corresponding inductive
clause in the proof of the theorem in (11).)
(3) ϕ = ψ → χ. (Again, the proof of this is just like the corresponding
inductive clause in (11).)
Proof of (16): Suppose ϕ is a tautology. Then by (13), every total assignment f is such that f (ϕ) = T . So for any model M and world w, the total
assignment fw determined by M and w is such that fw (ϕ) = T . But by
M
(15), fw (ϕ) = T iff |=M
w ϕ. So for every M and w, |=w ϕ. So ϕ is valid.
Proof of (12) (Alternative §2): Suppose we are given a set Γ∗ of quasiatomic formulas. Then we prove our theorem by induction on ϕ ∈
Fml ∗ (Γ∗ ∪ {⊥}). Base case: ϕ = p∗ . Suppose that f and f 0 both extend
f ∗ . Since f extends f ∗ , then f agrees with f ∗ on all the quasi-atomics
in Γ∗ . So f (p∗ ) = f ∗ (p∗ ), since p∗ ∈ Γ∗ . Similarly, f 0 (p∗ ) = f ∗ (p∗ ). So
f (p∗ ) = f 0 (p∗ ); i.e., f (ϕ) = f 0 (ϕ).
Induction cases: (1) Suppose that ϕ = ⊥. Then, by the definition of
subformula, ⊥ is a subformula of ϕ, since every formula is a subformula
of itself. So by hypothesis, f (⊥) = f 0 (⊥), and hence, f (ϕ) = f 0 (ϕ).
(2) Suppose ϕ = ¬ψ. We may assume, as an inductive hypothesis, that
the theorem holds for ψ, i.e., we may assume for the inductive hypothesis
that f (ψ) = f 0 (ψ). Exercise: Show that f (ϕ) = f 0 (ϕ).
(3) Suppose that ϕ = ψ → χ. We may assume as inductive hypotheses
both that f (ψ) = f 0 (ψ) and that f (χ) = f 0 (χ). Exercise: Show that
f (ϕ) = f 0 (ϕ).
Proof of (15) (Alternative §2): Fix ΛΩ , M, and w. Now pick an arbitrary
∗
formula ϕ and consider the basic assignment fw
of Ω∗ϕ determined by M
∗
and w. We first argue by induction on Fml (Ω∗ϕ ∪ {⊥}) that for every
∗
∗
ψ ∈ Fml ∗ (Ω∗ϕ ∪{⊥}), fw
(ψ) = T iff |=M
w ψ. Then we argue that fw (ϕ) = T
∗
∗
iff |=M
w ϕ on the grounds that ϕ ∈ Fml (Ωϕ ∪ {⊥}).
Induction on Fml ∗ (Ω∗ϕ ∪ {⊥}). Base case: ψ = p∗ . Since fw agrees
∗
∗
with fw
on all the quasi-atomic formulas in Ω∗ϕ , fw (p∗ ) = fw
(p∗ ).
∗
∗
∗
But by definition, fw
(p∗ ) = T iff |=M
w (p ). So fw (p ) = T iff
M
∗
M
|=w (p ); i.e., fw (ψ) = T iff |=w ψ.
83
Inductive cases: (a) ψ = ⊥. To prove our biconditional, we prove
conditionals in both directions. (⇒) By the definition of extended
assignments, fw (⊥) = F . So if fw (⊥) = T , then |=M
w ⊥ (by antecedent failure). (⇐) By the definition of truth at a world in a
M
model, 6|=M
w ⊥. So if |=w ⊥, then fw (⊥) = T (again by antecedent
failure). So assembling our two conditionals, fw (⊥) = T iff |=M
w ⊥,
M
i.e., fw (ψ) = T iff |=w ψ.
(b) ψ = ¬χ. (Exercise).
(c) ψ = χ → θ. (Exercise)
∗
∗
∗
(ϕ) = T iff |=M
Consequently, fw
w ϕ, since ϕ ∈ Fml (Ωϕ ∪ {⊥}).
Proof of (16) (Alternative §2): Suppose ϕ is a tautology. Then by (13),
for every f ∗ of the quasi-atomic subformulas in ϕ, f (ϕ) = T . But for
∗
∗
must
, and so fw
every M and w, there is a basic assignment function fw
determine an extended assignment fw which is such that fw (ϕ) = T . In
other words, for every model M and world w, fw (ϕ) = T . But by (14),
M
fw (ϕ) = T iff |=M
w ϕ. So, for every model M and world w, |=w ϕ. So ϕ is
a valid.
Proof of (32): Suppose ψ is a tautological consequence of ϕ1 , . . . , ϕn .
Then by (31.2), ϕ1 → . . . → ϕn → ψ is a tautology. Hence, by (16),
ϕ1 → . . . → ϕn → ψ is valid. And by hypothesis, each of ϕ1 , . . . , ϕn is
valid. So by n applications of the fact (30) that Modus Ponens preserves
validity, it follows that ψ is also valid.
Chapter Four
Proof of (38): (⇒) Suppose Γ is a modal logic, i.e., that Γ contains every
tautology and is closed under Modus Ponens. To show that Γ is closed under RPL, suppose that that ϕ1 , . . . , ϕn ∈ Γ and ψ is a tautological consequence of ϕ1 , . . . , ϕn . Since ψ is a tautological consequence of ϕ1 , . . . , ϕn ,
ϕ1 → . . . → ϕn → ψ is a tautology, by (31.2). So ϕ1 → . . . → ϕn → ψ is
an element of Γ. But since Γ is closed under MP, n applications of this
rule yields that ψ ∈ Γ.
(⇐) Suppose Γ is closed under RPL. (a) Suppose ϕ is a tautology.
Then by (31.4), ϕ is a tautological consequence of ϕ1 , . . . , ϕn when n = 0.
But when n = 0, ϕ1 , . . . , ϕn are all in Γ. So since Γ contains all the
tautological consequences of any ϕ1 , . . . , ϕn whenever ϕ1 , . . . , ϕn ∈ Γ, it
84
contains ϕ. (b) Suppose, next that ϕ, ϕ → ψ ∈ Γ. By (31.1), ψ is a
tautological consequence of ϕ and ϕ → ψ. So ψ ∈ Γ, since Γ is closed
under RPL. Thus, Γ is closed under Modus Ponens.
Proof of (43.2): Clearly, there is an effective method for telling whether
ϕ ∈ ∅. And given the previous Remark , there is an effective method for
telling whether a formula ϕ is a tautological consequence of ϕ1 , . . . , ϕn . So
by (42), we have to show that ϕ ∈ PL iff there is a sequence of formulas
hϕ1 , . . . , ϕn i, with ϕ = ϕn , such that each member of the sequence is
either (a) a member of the empty set ∅ of axioms, or (b) is the conclusion
by RPL of previous members of the sequence.
(⇒) Suppose ϕ ∈ PL. Then ϕ is a tautology. To show that there
is a sequence meeting the conditions of the theorem, we have only to
show that there is a sequence hϕ1 , . . . , ϕn i (with ϕn = ϕ) every member
of which is the conclusion by RPL of previous members (since none of
the members of the sequence are ever in the empty set ∅ of axioms). Let
the sequence be just ϕ itself. So to show that ϕ is the conclusion by
RPL of the previous members in the sequence, we must establish that ϕ
is a tautological consequence of the empty set ∅ of hypotheses. But ϕ is
tautology, and so by (31.4), it is such a consequence.
(←) Assume that there is a sequence of formulas hϕ1 , . . . , ϕn i (with
ϕn = ϕ) meeting the conditions of the theorem. Thus, each ϕi in the
sequence must be a conclusion by RPL of previous members in the sequence. We want to show that ϕ ∈ PL. We prove this by induction on
the length of the sequence. Suppose that the sequence has length 1. Then
there are no previous members of the sequence. By hypothesis, then, ϕ
is a conclusion by RPL of the empty set ∅. So ϕ must be a tautological
consequence of ∅, and so by (31.4), ϕ is a tautology. Hence ϕ ∈ PL.
Inductive case: Suppose the sequence has length i < n. So there is a sequence hϕ1 , . . . , ϕi (= ϕ)i such that each member is the conclusion by RPL
of previous members of the sequence. So, in particular, ϕi must be a tautological consequence of previous members ϕj1 , . . . , ϕjk (1 ≤ j1 ≤ jk < i).
So consider the sequences: hϕ1 , . . . , ϕj1 i, hϕ1 , . . . , ϕj2 i, . . . , hϕ1 , . . . , ϕjk i.
Now since each of these sequences has a length less than n, we may apply the inductive hypothesis to each sequence, concluding overall that
ϕj1 , . . . , ϕjk are members of PL, and so all are tautologies. But then ϕi
is a tautological consequence of tautologies, and so by (31.3), ϕi , i.e., ϕ,
is itself a tautology, and so a member of PL.
85
Proof of (45): Suppose that Γ `Σ ϕ and that ψ is a tautological consequence of ϕ. Since Γ `Σ ϕ, there are ϕ1 , . . . , ϕn ∈ Γ such that `Σ ϕ1 →
. . . → ϕn → ϕ. Now since ψ is a tautological consequence of ϕ, ϕ → ψ is
a tautology (by (31.1) ). So `Σ ϕ → ψ, since Σ contains every tautology.
But the following is also a tautology:
[ϕ1 → . . . → ϕn → ϕ] → [(ϕ → ψ) → [ϕ1 → . . . → ϕn → ψ]]
So this displayed formula is an element of Σ. But the antecedent of this
displayed formula, and the antecedent of its consequent, are both elements
of Σ, and since Σ is closed under MP, the consequent of the consequent
must be an element of Σ as well. So `Σ ϕ1 → . . . → ϕn → ψ, and thus
∃ϕ1 , . . . , ϕn ∈ Γ such that `Σ ϕ1 → . . . → ϕn → ψ. Hence, Γ `Σ ψ.
Proof of (47): Suppose Γ `Σ ϕ1 and . . . and Γ `Σ ϕn , and ψ is a tautological consequence of ϕ1 , . . . , ϕn . Since Γ `Σ ϕ1 , there are χ11 , . . . , χ1m ∈ Γ
such that `Σ χ11 → . . . → χ1m → ϕ1 ; . . . ; and since Γ `Σ ϕn , there
are χn1 , . . . , χnm ∈ Γ such that `Σ χn1 → . . . → χnm → ϕn . Now since ψ
is a tautological consequence of ϕ1 , . . . , ϕn , ϕ1 → . . . → ϕn → ψ is a
tautology (by (31.2) ). So `Σ ϕ1 → . . . → ϕn → ψ. Thus far, we know
the following are theorems of Σ:
A1 : χ11 → . . . → χ1m → ϕ1
..
.
An : χn1 → . . . → χnm → ϕn
B: ϕ1 → . . . → ϕn → ψ
But the following is a tautology:
A1 → . . . → An → B →
χ11 → . . . → χ1m → . . . → χn1 → . . . → χnm → ψ
Since Σ is closed under MP, and A1 , . . . , An , and B are elements of Σ,
n + 1 applications of MP yields the following as a theorem of Σ:
χ11 → . . . → χ1m → . . . → χn1 → . . . → χnm → ψ
Since this is a theorem of Σ, and all of the χij s are elements of Γ, it follows
by definition that Γ `Σ ψ.
Proof of Exercise 1 in (49): (⇒) Assume that Γ is deductively-closedΣ .
We want to show: (a) that Γ is a modal logic, and (b) that Σ ⊆ Γ. To
86
show (a), we need to show both (i) that Γ contains every tautology, and
(ii) that Γ is closed under MP. To show (i), suppose that ϕ is a tautology.
Then by (40), `Σ ϕ. And by (46.2), Γ `Σ ϕ. So by the fact that Γ is
deductively closed, ϕ ∈ Γ. To show (ii), suppose that ϕ → ψ and ϕ and in
Γ. Then by (46.5), both Γ `Σ ϕ → ψ and Γ `Σ ϕ. So by (48.7), Γ `Σ ψ.
And by the deductive closure of Γ, ψ ∈ Γ.19 To show (b), assume ϕ ∈ Σ,
i.e., `Σ ϕ. By (46.2), ϕ is derivableΣ from every set of sentences. So
Γ `Σ ϕ. Since Γ is deductively-closedΣ , ϕ ∈ Γ.
(⇐) Assume that Γ is a Σ-logic. So Γ contains every tautology, Γ is
closed under MP, and Σ ⊆ Γ. Assume Γ `Σ ϕ (to show ϕ ∈ Γ). Then
∃ϕ1 , . . . , ϕn ∈ Γ such that `Σ ϕ1 → . . . → ϕn → ϕ. So, ϕ1 → . . . →
ϕn → ϕ ∈ Σ. But since Σ ⊆ Γ, ϕ1 → . . . → ϕn → ϕ ∈ Γ. But since
ϕ1 , . . . , ϕn ∈ Γ, and Γ is closed under MP, ϕ ∈ Γ.
Proof of Exercise 2 in (49): (a) Suppose that Γ `Σ ϕ and |=M
w Σ ∪ Γ. So
∃ϕ1 , . . . , ϕn ∈ Γ such that `Σ ϕ1 → . . . → ϕn → ϕ. Since ϕ1 → . . . →
ϕn → ϕ ∈ Σ, ϕ1 → . . . → ϕn → ϕ ∈ Σ ∪ Γ. So |=M
w ϕ1 → . . . → ϕn → ϕ,
by hypothesis. But since ϕ1 , . . . , ϕn ∈ Γ, ϕ1 , . . . , ϕn ∈ Σ ∪ Γ. So |=M
w ϕ1
M
and . . . and |=w ϕn . So by n applications of the fact that MP preserves
truth at a world in a model (see the Remark in (30)), |=M
w ϕ.
(b) To prove (i) that {ϕ | Γ `Σ ϕ} is a modal logic, assume that ψ is a
tautology (to show ψ ∈ {ϕ | Γ `Σ ϕ}). So by (40.1), ψ ∈ Σ, and by (46.2),
Γ `Σ ψ. So ψ ∈ {ϕ | Γ `Σ ϕ}, and thus, this set contains every tautology.
To see that {ϕ | Γ `Σ ϕ} is closed under MP, assume that it contains both
ψ → χ and ψ. So Γ `Σ ψ → χ and Γ `Σ ψ. By (48.7), Γ `Σ ψ, and so
ψ ∈ {ϕ | Γ `Σ ϕ}. Thus, this set is closed under MP, and so it is a modal
logic. To prove (ii) that {ϕ | Γ `Σ ϕ} contains Σ∪Γ, assume that ψ ∈ Σ∪Γ
(to show Γ `Σ ψ). Then either ψ ∈ Σ or ψ ∈ Γ. If the former, then `Σ ψ,
and so by (46.2), Γ `Σ ψ. If the latter, then by (46.5), Γ `Σ ψ. To prove
(iii) that {ϕ | Γ `Σ ϕ} is a subset of any logic Σ0 containing Σ ∪ Γ, suppose
that Σ0 is a logic containing Σ∪Γ. Suppose that ψ ∈ {ϕ | Γ `Σ ϕ} (to show
ψ ∈ Σ0 ). So Γ `Σ ψ. So ∃ψ1 , . . . ψn ∈ Γ such that `Σ ψ1 → . . . → ψn → ψ.
Since ψ1 , . . . , ψn ∈ Γ and ψ1 → . . . → ψn → ψ ∈ Σ, we know that ψ1 , . . . ,
ψn , and ψ1 → . . . → ψn → ψ are all in Σ ∪ Γ. But then ψ1 , . . . , ψn , and
ψ1 → . . . → ψn → ψ are all in Σ0 . So since Σ0 is a modal logic, it is closed
19 An
alternate proof of (a) is to show Γ is closed under RPL: assume that
ϕ1 , . . . , ϕn ∈ Γ, and that ψ is a tautological consequence of ϕ1 , . . . , ϕn . Then, by
(46.5), Γ `Σ ϕ1 , . . . , Γ `Σ ϕn . So by (47), Γ `Σ ψ. But, by hypothesis, Γ is
deductively-closedΣ . So ψ ∈ Γ.
87
under MP. Hence, ψ ∈ Σ0 .
Proof of (56): (⇒) Assume Γ `Σ ϕ and that both MaxConΣ (∆) and
Γ ⊆ ∆. Then by (46.7), ∆ `Σ ϕ. So by (54.1), ϕ ∈ ∆. (⇐) Assume that
ϕ is an element of every MaxConΣ (∆) of which Γ is a subset. For reductio,
suppose that Γ 6 `Σ ϕ. Then, by (52.8), ConΣ (Γ ∪ {¬ϕ}). So by (55), there
is a MaxConΣ (∆0 ) of which Γ ∪ {¬ϕ} is a subset. By this last fact, both
Γ ⊆ ∆0 and {¬ϕ} ⊆ ∆0 , and so ¬ϕ ∈ ∆0 . But since ϕ is a member of
every MaxConΣ (∆) of which Γ is a subset, ϕ ∈ ∆0 , contradicting (54.4)
for MaxConΣ (∆).
Proof of Exercise in (57): Suppose throughout that ConΣ (Γ). (⇒) Assume that for every ϕ, if ConΣ (Γ ∪ {ϕ}), then ϕ ∈ Γ. To show that
Max(Γ), we show that if ψ 6∈ Γ, then ¬ψ ∈ Γ. So assume ψ 6∈ Γ. Then,
by our assumption, CønΣ (Γ ∪ {ψ}). So by (52.9), Γ `Σ ¬ψ. Assume, for
reductio, that ¬ψ 6∈ Γ. Then, again by assumption, CønΣ (Γ ∪ {¬ψ}). So
by (52.8), Γ `Σ ψ. So both Γ `Σ ¬ψ and Γ `Σ ψ, contradicting the consistency of Γ. So ¬ψ ∈ Γ (⇐) Assume Max(Γ). Assume ConΣ (Γ ∪ {ψ}). So
by (52.9), Γ 6 `Σ ¬ψ. Now suppose for reductio that ψ 6∈ Γ. Then, since
Max(Γ), ¬ψ ∈ Γ. So by (46.5), Γ `Σ ¬ψ, which is a contradiction. So
ψ ∈ Γ.
Proof of (59): (⇒) Suppose Σ is a normal modal logic. Since it is a modal
logic, it contains every tautology and is closed under MP, and since it is
normal, it contains the instances of the axiom K and is closed under RN.
We want to show that if `Σ ϕ1 → . . . → ϕn → ϕ, then `Σ ϕ1 →
. . . → ϕn → ϕ. We prove this by induction: (a) Base case: When
n = 0, then we have to show that if `Σ ϕ, then `Σ ϕ. But this follows
immediately, since Σ is closed under RN. (b) Assume `Σ ϕ1 → . . . →
ϕn → ϕ. Now our inductive hypothesis is: if `Σ ϕ1 → . . . → ϕn−1 → ψ,
then `Σ ϕ1 → . . . → ϕn−1 → ψ. So by our inductive hypothesis,
`Σ ϕ1 → . . . → ϕn−1 → (ϕn → ϕ). But since Σ contains all the
instances of the axiom K, we know `Σ (ϕn → ϕ) → (ϕn → ϕ). So
given Σ contains all tautologies and is closed under MP, `Σ ϕ1 → . . . →
ϕn−1 → (ϕn → ϕ), i.e., `Σ ϕ1 → . . . → ϕn → ϕ. (⇐) Suppose
Σ is closed under RK. To show Σ is normal, we show (a) that it contains
the (instances of the) schema K and (b) is closed under RN. (a) Since
(ϕ → ψ) → ϕ → ψ is a tautology, `Σ (ϕ → ψ) → ϕ → ψ. So by RK,
`Σ (ϕ → ψ) → ϕ → ψ, i.e., `Σ (ϕ → ψ) → (ϕ → ψ). (b) RN is
the special case of RK where n = 0.
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Proof of (60.1): Suppose Σ is normal. To simplify the proof, we note
that since Σ is a logic, it is closed under RPL. By the definition of ♦ψ
in the Remark in (3), we know that Σ contains ♦ψ ↔ ¬¬ψ (since Σ
contains all tautologies and this just abbreviates the tautology ¬¬ψ ↔
¬¬ψ). Since Σ contains all the tautologies of this form, it contains
♦¬ϕ ↔ ¬¬¬ϕ (substituting ¬ϕ for ψ). Thus, by closure under RPL, Σ
will contain the following biconditional obtained by negating both sides:
¬♦¬ϕ ↔ ¬¬¬¬ϕ. But since ¬¬¬¬ϕ ↔ ¬¬ϕ is a tautology, closure
under RPL also requires that Σ contain ¬♦¬ϕ ↔ ¬¬ϕ. However, in
order to show that Σ contains ¬♦¬ϕ ↔ ϕ, we have to show that Σ
contains ¬¬ϕ ↔ ϕ. But to do this, we need the fact that Σ is closed
under RK, for given ϕ → ¬¬ϕ and ¬¬ϕ → ϕ are tautologies and hence
members of Σ, it follows by RK that both ϕ → ¬¬ϕ and ¬¬ϕ → ϕ
are members of Σ. And so the biconditional ¬¬ϕ ↔ ϕ will be a
member of Σ. Thus, by closure under RPL, Σ contains ¬♦¬ϕ ↔ ϕ.
Proof of Exercise 4 in (60): Let Σ be normal and suppose that `Σ ψ ↔ ψ 0 .
We show that `Σ ϕ[ψ 0 /ψ]. Suppose that ϕ and ψ are the same sentence.
Then ϕ[ψ 0 /ψ] is either ϕ (when zero occurrences of ψ are replaced by ψ 0 )
or ψ 0 (when ϕ, i.e., ψ, is replaced by ψ 0 ). In either case, `Σ ϕ[ψ 0 /ψ].
So assume that ϕ and ψ are distinct. Then we complete the proof by
induction on the complexity of ϕ. Base case: Suppose ϕ = p.
Inductive cases: (a) Suppose ϕ = ⊥.
(b) Suppose that ϕ = χ → θ. For inductive hypotheses, we may
assume `Σ χ ↔ χ[ψ 0 /ψ] and `Σ θ ↔ θ[ψ 0 /ψ].
(c) Suppose that ϕ = χ. For the inductive hypothesis, we assume `Σ
χ ↔ χ[ψ 0 /ψ]. Use RE and then the fact that (χ[ψ 0 /ψ]) = (χ)[ψ 0 /ψ].
Proof of (63): Clearly, there is an effective way of telling whether a formula is a tautology or an instance of the K schema. Moreover, the rules
MP and RN can be applied effectively. So given the definitions in (42),
we have to show that ϕ ∈ K iff there is a proof of ϕ from the tautologies
and the K axiom using the rules MP and RN, i.e., we have to show that
`K ϕ iff there is a sequence of formulas ϕ1 , . . . , ϕn (= ϕ) such that every
member of the sequence either (a) is a tautology or an instance of K, or
(b) follows from previous members by MP or RN.
Proof of (65): Since there is an effective method for telling whether a
formula is a tautology or an instance of K, S1 , . . . , Sn , and the rules MP
and RN can be applied effectively, it suffices to show, by the definitions
89
in (42), that ϕ ∈ KS1 . . .Sn iff there is a proof of ϕ from the tautologies
and axioms K, S1 , . . . , Sn using the rules of inference MP and RN; i.e.,
it suffices to show that `KS1 ...Sn ϕ iff there is a sequence of formulas
hϕ1 , . . . , ϕn i (with ϕ = ϕn ) every member of which either (a) is a tautology
or an instance of K, or S1 , or . . . , or Sn , or (b) follows from previous
members of the sequence by MP or RN.
Proof of (66): Suppose that Σ is normal and that Γ `Σ ϕ. Then
∃ϕ1 , . . . , ϕn ∈ Γ such that `Σ ϕ1 → . . . → ϕn → ϕ. Since Σ is normal, it follows by (59) that `Σ ϕ1 → . . . → ϕn → ϕ. But since
ϕ1 , . . . , ϕn ∈ Γ, ϕ1 , . . . , ϕn ∈ {ψ | ψ ∈ Γ}. So, by the definition of
deducibility, {ψ | ψ ∈ Γ} `Σ ϕ.
Proof of (68): (⇒) Assume ϕ ∈ Γ, MaxConΣ (∆) and {ψ | ψ ∈ Γ} ⊆ ∆,
for arbitrary ∆. Then, by definition, ϕ ∈ {ψ | ψ ∈ Γ}. So, by hypothesis,
ϕ ∈ ∆0 . (⇐) We use (67), though a proof can be constructed using (66)
as well. Assume that every MaxConΣ (∆) such that {ψ | ψ ∈ Γ} ⊆ ∆
is also such that ϕ ∈ ∆. Then by (56), {ψ | ψ ∈ Γ} `Σ ϕ. So by (67),
Γ `Σ ϕ. And by (54.1), ϕ ∈ Γ, since Γ is MaxConΣ .
Proof of the Exercise in (68) : (⇐) Assume that every MaxConΣ (∆)
such that {ψ | ψ ∈ Γ} ⊆ ∆ is also such that ϕ ∈ ∆. Then by (56),
{ψ | ψ ∈ Γ} `Σ ϕ. So by (66), {χ | χ ∈ {ψ | ψ ∈ Γ}} `Σ ϕ, i.e.,
{χ | χ ∈ Γ} `Σ ϕ. Since {χ | χ ∈ Γ} ⊆ Γ, it follows by (46.7) that
Γ `Σ ϕ. So by (54.1), ϕ ∈ Γ.
Proof of (69): (⇒) Assume that for every ϕ, if ϕ ∈ Γ, then ϕ ∈ ∆.
Assume also that ψ ∈ ∆ (to show ♦ψ ∈ Γ). If ψ ∈ ∆, then ¬ψ 6∈ ∆, by
(54.4). So by hypothesis, ¬ψ 6∈ Γ. So by the definition of ♦ψ, ♦ψ ∈ Γ.
(⇐) Assume that for every ϕ, if ϕ ∈ ∆, then ♦ϕ ∈ Γ. Assume that
ψ ∈ Γ (to show ψ ∈ ∆). Since Σ is normal, Σ contains ψ ↔ ¬♦¬ψ,
by (60). Since MaxConΣ Γ, Σ ⊆ Γ, by (54.2). Consequently, Γ must also
contain ψ ↔ ¬♦¬ψ. So since Γ contains ψ by assumption, we know
¬♦¬ψ ∈ Γ, by (54.8), and so ♦¬ψ 6∈ Γ, by (54.4). So by hypothesis,
¬ψ 6∈ ∆. But then ψ ∈ ∆, by (54.4).
Proof of (70): By (54.4), we know that ♦ϕ ∈ Γ (i.e., ¬¬ϕ ∈ Γ) iff
¬ϕ 6∈ Γ. But by (68), ¬ϕ 6∈ Γ iff there is a MaxConΣ (∆) such that
both (i) {ψ | ψ ∈ Γ} ⊆ ∆ and (ii) ¬ϕ 6∈ ∆. But by the Remark in (69),
we know that clause (i) is equivalent to clause (a) in the statement of the
theorem. And by (54.4), we know that clause (ii) is equivalent to clause
90
(b) in the statement of the theorem. Thus, substituting equivalents, we
may infer ♦ϕ ∈ Γ iff there is a MaxConΣ (∆) such that both {♦ψ | ψ ∈
∆} ⊆ Γ and ϕ ∈ ∆.
Chapter Five
Proof of Lemma in (71.2): Assume (i), (ii), and (iii). Assume further
that ϕ is a theorem of Σ (to show that ϕ is valid with respect to C).
Since Σ is axiomatized by Γ and the rules R1 , R2 , . . . , it follows by (42)
that there is a proof of ϕ from Γ using the rules, i.e., that there is a
sequence of formulas ϕ1 , . . . , ϕn (with ϕ = ϕn ) every member of which is
either a member of Γ or a conclusion by a rule of previous members in
the sequence. We now argue by induction on the length of a proof that
ϕ is valid with respect to C.
Base case: Suppose that the proof of ϕ is a one member sequence,
namely, ϕ itself. Then either ϕ is a member of Γ or follows from previous
members of the sequence (of which there are none) by a rule. If ϕ is a
member of Γ, then ϕ is valid with respect to C, by hypothesis. If ϕ follows
from the empty set of previous members by a rule of inference, then since
the rules all preserve validity with respect to C, ϕ is valid with respect
to C.
Inductive case: Suppose that the proof of ϕ is an n member sequence.
Then ϕ (= ϕn ) must follow from previous members, say ϕj1 , . . . , ϕjk (1 ≤
j1 ≤ jk < n), by one of the rules. But consider the following sequence of
sequences: hϕ1 , . . . , ϕj1 i, hϕ1 , . . . , ϕj2 i, . . . , hϕ1 , . . . , ϕjk i. Now since each
of these sequences has a length less than n, and so we may apply the
inductive hypothesis to each sequence, concluding overall that C |= ϕj1 ,
C |= ϕj2 , . . . , C |= ϕjk . And since ϕ follows from ϕj1 , . . . , ϕjk by R, and
R preserves validity with resepct to C, it follows that C |= ϕ.
Proof of (75): Let CF be the class of all frames and note by (61) that
{ψ | CF |= ψ} is a normal logic. Since K is the smallest normal modal
logic, K is a subset of {ψ | CF |= ψ}, for any class of frames CF . So
the theorems (i.e., members) of K are all members of this such set; i.e.,
if `K ϕ, then CF |= ϕ. So, K is sound with respect to the class of all
frames.
Proof of (79): By induction on ΛΩ with respect to an arbitrarily chosen
Σ
Σ
world w. Base case: Suppose that ϕ = p. By (6.6), |=M
w p iff w ∈ V (p).
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But by (78.3), w ∈ VΣ (p) iff w ∈ {w0 ∈ WΣ | p ∈ w0 }, i.e., iff p ∈ w. So
Σ
MΣ
|=M
w p iff p ∈ w. So |=w ϕ iff ϕ ∈ w.
Σ
Inductive cases: (i) Suppose that ϕ = ⊥. By (6.2), 6|=M
w ⊥. So by
Σ
antecedent failure, if |=M
w ⊥, ⊥ ∈ w. Moreover, by (54.3) and the fact
that w is MaxConΣ (78.1), ⊥ 6∈ w. So by antecedent failure, if ⊥ ∈ w,
Σ
MΣ
MΣ
then |=M
w ⊥. Thus, |=w ⊥ iff ⊥ ∈ w; i.e., |=w ϕ iff ϕ ∈ w.
Σ
Σ
(ii) Suppose ϕ = ¬ψ. By (6.3), |=M
¬ψ iff 6|=M
ψ. But, by our
w
w
Σ
Σ
M
inductive hypothesis, |=M
ψ
iff
ψ
∈
w.
So
|=
¬ψ
iff
ψ
6∈ w. Since w
w
w
Σ
is MaxConΣ (by (78.1)), ψ 6∈ w iff ¬ψ ∈ w (by (54.4)). So |=M
w ¬ψ iff
Σ
¬ψ ∈ w, i.e., |=M
w ϕ iff ϕ ∈ w.
Σ
MΣ
MΣ
(iii) Suppose ϕ = ψ → χ. By (6.4), |=M
w ψ → χ iff 6|=w ψ or |=w χ.
But our inductive hypotheses are:
Σ
|=M
w ψ iff ψ ∈ w
MΣ
|=w χ iff χ ∈ w
Σ
So |=M
w ψ → χ iff either ψ 6∈ w or χ ∈ w. But since w is MaxConΣ
Σ
(78.1), either ψ 6∈ w or χ ∈ w iff ψ → χ ∈ w, by (54.7). So |=M
w ψ → χ
Σ
iff ψ → χ ∈ w, i.e., |=M
w ϕ iff ϕ ∈ w.
Σ
(iv) Suppose ϕ = ψ. By (6.5), |=M
ψ iff for every w0 , if (a)
w
Σ
Σ
0
M
R ww , then (b) |=w0 ψ. By (78.2), (a) holds iff {χ | χ ∈ w} ⊆ w0 . By
our inductive hypothesis, (b) holds iff ψ ∈ w0 . So, assembling our results
Σ
0
0
0
so far, |=M
w ψ iff for every w , if {χ | χ ∈ w} ⊆ w , then ψ ∈ w , i.e,
0
0
iff ψ is an element of every world w such that {χ | χ ∈ w} ⊆ w . But
since w and w0 are MaxConΣ and Σ is normal, it follows by (68) that
Σ
MΣ
this condition holds iff ψ ∈ w. Thus, |=M
w ψ iff ψ ∈ w, i.e, |=w ϕ
iff ϕ ∈ w.
Σ
Proof of (80): By (79), for all w ∈ WΣ , [ |=M
w ϕ iff ϕ ∈ w]. It follows
Σ
Σ
ϕ]
iff
[for
every
w,
ϕ ∈ w], i.e., that |=M ϕ
that: [for every w, |=M
w
iff for every w, ϕ ∈ w. But by (78.1), every world w is MaxConΣ . So,
Σ
|=M ϕ iff for every MaxConΣ (Γ), ϕ ∈ Γ. But by (57), if follows that
Σ
|=M ϕ iff `Σ ϕ.
92